“Onigiri” (also called “nigirimeshi” and “omusubi”) are Japanese rice balls. The rice balls have various fillings, such as pickled ume (umeboshi), and are usually wrapped in nori (seaweed). The food is portable and has been included in many boxed lunches (such as bento).
 
“NIGIRI-MESHI, n. Cold rice eaten by making it into a ball” was printed in the book Japanese-English and English-Japanese Dictionary (1873). “A ball of rice, called ‘Nigirimeshi’” was printed in The Ladies’ Repository in December 1875. “Rice-balls, n. musubi; nigiri-meshi” was printed in the book An English-Japanese Dictionary of the Spoken Language (1904). “Nigiri-zushi small, barely-warm rice-balls with fish slices on top; nigiru means clutched in a fist” was printed in Contemporary Japan (1951).
   
“Box lunches of ‘onigiri and yakitori’ (Japanese rice cakes and barbecued chicken)” was printed in the Chicago (IL) Daily Tribune on August 5, 1962. A recipe for onigiri was printed in The Arizona Republic (Phoenix, AZ) on November 1, 1967.
 
     
Wikipedia: Onigiri
O-nigiri (お握り or 御握り; おにぎり), also known as o-musubi (お結び; おむすび), nigirimeshi (握り飯; にぎりめし), rice ball, is a Japanese food made from white rice formed into triangular or cylindrical shapes and often wrapped in nori (seaweed). Traditionally, an onigiri is filled with pickled ume (umeboshi), salted salmon, katsuobushi, kombu, tarako, or any other salty or sour ingredient as a natural preservative. Most Japanese convenience stores stock their onigiri with various fillings and flavors. There are even specialized shops which only sell onigiri to take out. Due to the popularity of this trend in Japan, onigiri has become a popular staple in Japanese restaurants worldwide.
 
Despite common misconceptions, onigiri is not a form of sushi. Onigiri is made with plain rice (sometimes lightly salted), while sushi is made of rice with vinegar, sugar and salt. Onigiri makes rice portable and easy to eat as well as preserving it, while sushi originated as a way of preserving fish.
       
Google Books
Japanese-English and English-Japanese Dictionary
By James Curtis Hepburn
New York, NY: A. D. F. Randolph & Company
1873
Pg. 191:
NIGIRI-MESHI, n. Cold rice eaten by making it into a ball.
         
Google Books 
10 March 1875, The Japan Mail, pg. 134, col. 1:
The “sandwich” of the theatre, called maku-no-ouchi, consists of handfuls of rice squeezed, nigiri-meshi, and a vegetable stew nishime — as a relish.
 
Google Books
December 1875, The Ladies’ Repository, “The Old Wood-cutter’s Luck (A Japanese Fairy Tale),” pg. 563, col. 1:
Well, one morning he went to the mountain, as usual, to cut wood, carrying with him his noon-day meal, which was only a ball of rice, called “Nigirimeshi,” and which to this day the working-men eat for their lunch.
 
Google Books
An English-Japanese Dictionary of the Spoken Language
By Ernest Miles Sataw and Ishibashi Masakata
Third Edition revised and enlarged by E. M. Hobart-Hampden and harold G. Parlett
Tokyo, Japan: Printed by the Shueisha
1904
Pg. 733:
Rice-balls, n. musubi; nigiri-meshi.
       
Google Books
The Nightless City
Or the “History of the Yoshiwara Yūkwaku”
By Joseph Ernest De Becker
Yokohama, Japan: Max Nössler & Company
1905
Pg. 150:
If it has grown on the lower eyelid, you must receive some nigiri-meshi (boiled rice rolled up into a ball-like shape) from a person in an inferior station in life and eat the same: the nigiri-meshi must be received from outside of the window, the
 
Google Books
The Problem of Japan
By Malcolm Duncan Kennedy
London, UK: Nisbet & Co. Ltd.
1935
Pg. 239:
“Food, when available, consisted of mugi-meshi (rice and barley mixed) and nigiri-meshi (rice- balls).”
     
Google Books
Contemporary Japan
Tokyo, Japan: Foreign Affairs Association of Japan
1951
Pg. 89:
nigiri-zushi small, barely-warm rice-balls with fish slices on top; nigiru means clutched in a fist.
   
Google Books
I Was Defeated
By Yoshio Kodama
Tokyo, Japan: R. Booth and T. Fukuda
1951
Pg. 152:
I suddenly noticed beside the seats of the pilots the food of these young boys — “nigiri- meshi” (rice-balls), two for each man, wrapped separately in the pliant skins of bamboo sprouts. These “nigiri-meshi” wrapped in the skins of bamboo sprouts suddenly took me back to the days of my boyhood when I too used to carry such lunches on outings and school picnics.
     
11 April 1953, Lincoln (NE) Star, pg. 6, col. 6:
... nigirimeshi (rice balls) ...
(A Japanese dinner is described.—ed.)
 
Google Books
Mock Joya’s Things Japanese
By Moku Jōya
Tokyo, Japan: Tokyo News Service
1960
Pg. 257:
Nigiri-meshi is a ball of rice made by pressing rice between two palms, and is used as luncheon on picnics or trips. Generally umeboshi, pickled apricot, is placed in the center of the ball. Nigiri-meshi is made into various shapes, some round, and some triangular. Salt, or salt and sesame, is sprinkled over the outside of the ball, but it is also often covered with nori (dried seaweed).
 
5 August 1962, Chicago (IL) Daily Tribune, “Annual Picnic Plans Are Told by Japanese,” pt. 1, pg. 2, col. 6:
The Women’s Fellowship of Christ Congressional church, 701 Buckingham pl., has prepared box lunches of “onigiri and yakitori” (Japanese rice cakes and barbecued chicken) for sale.
   
21 October 1964, The Christian Science Monitor (Boston, MA), “Olympic Mother” by Stuart Griffin, pg. 4, col. 5:
Her “secret weapon” is in being a good cook, especially of a unique Japanese dish, rice balls wrapped in seasoned seaweed, called “Onigiri.”
   
Google Books
The Golden Guide to South and East Asia, Volume 6
By Daniel Wolfstone
Hong Kong: Far Eastern Economic Review Limited
1967
Pg. 246:
Common lunches for travellers, students of workers are onigiri (rice balls wrapped in dried seaweed nori with a salted plum inside) or bento (cold rice in a wooden or lacquer box, topped with fish, vegetable, pickles, chicken, fish paste or almost anything).
 
1 November 1967, The Arizona Republic (Phoenix, AZ), “Japanese Floral Arrangements, Prints on Exhibit at PAM,” pg. 16-E, col. 3:
ONIGIRI
(Rice Balls)
1 cup rice
Water
Seaweed (available in Chinese specialty shops)
 
Wash rice thoroughly by rinsing many times. add about 1 1/4 cups water and soak for at least an hour before cooking. Bring to a boil. Immediately reduce heat to very low; keep rice covered; steam 20 to 30 minutes. Remove from heat.
 
Place rice in another pan; fan it until it has cooled. make oval balls about 3-inches long. Cut seaweed about 2-inches wide and wrap rice balls, allowing rice to show out either end of the seaweed wrap.
 
10 February 1968, Rockford (IL) Morning Star, “Olympic Sidelights,” pg. B2, cols. 1-2:
GRENOBLE, France (AP)—Japan’s Winter Olympic athletes say the French cooking in the Olympic Village is great but they still went wild about the present from the Japanese ambassador in Paris: Authentic Japanese mizo siro (soybean soup) and onigiri (rice balls).
 
25 July 1970, The Oregonian (Portland, OR), “Obon Festival Slated By Ontario Buddhists” by Chris Moore, pg. 11, col. 2:
Oriental treasures are now exhibited and a dinner featuring such dishes as teriyaki chicken, shrimp tempura, noodles and onigiri (rice cakes) will be served between 5 and 8 p.m. in the church basement.
 
6 October 1974,

(Honolulu, HI), pg. F-40, col. 2 ad:
ONIGIRI MAKER
Make rice balls the fast and easy way with this plastic onigiri maker. Just fill it with rice, press, and in a few seconds they’re ready.
Reg. 69c ... SALE 49c
(Shirokiya.—ed.)
 
Google Books
A Taste of Japan: Food Fact and Fable:
What the People Eat: Customs and Etiquette
By Donald Richie
Tokyo ; New York: Kodansha International, New York, NY: Harper & Row
1985
Pg. 71:
The idea of eating onigiri outside still persists and the majority of rice balls consumed in today’s Japan are enjoyed on park benches, lawns, or on real picnics. Perhaps the idea originally was that since rice balls are so portable they make ideal travel food. Their portability also perhaps accounts for their variations.
 
The most common rice ball is the nori musubi.
   
Google Books
Outlook on Japan
By Nihon Kōtsū Kōsha.
Tōkyō, Japan: Japan Travel Bureau
1989
Pg. 148:
The Japanese equivalent of the sandwich is the “onigiri”, or rice ball. “Onigiri” are often used in boxed lunches, for taking on picnics, for camping, or whenever a packed meal is required. They consist of rice formed into a ball with a filling such as pickled plum, salted salmon, salted cod roe or dried bonito shavings, wrapped in a sheet or “nori” (dried seaweed).
   
Google Groups: rec.food.recipes
Japanese Rice Balls (Onigiri)
Kai Hurada
4/26/07
This very good rice ball.
Japanese Rice Balls (Onigiri)
 
4 cups medium grain rice
1 envelope bonito shavings
5 seasoned plums (umeboshi)
4 ounces salmon
2 sheets nori (seaweed wrappers)
2 tsps. salt
 
Rinse salmon and pat dry with a paper towel. Sprinkle with salt. Place on a rack over a cookie sheet or small dish and bake in 400 degree oven for an hour to dry it out. Cook rice according to package directions (i.e. without adding fat) so that it’s sticky. Prepare three bowls. When salmon is done, ground the fillet up into small pieces with a fork or mortar and pestle and place in the first bowl. Mash seasoned plums with a fork and place in second bowl. Empty contents of one small bonito shaving envelope into the third bowl. Divide rice into three portions, and add one portion to each bowl. Mix lightly to combine with seasoning ingredient. Wet hands slightly. Dip one finger tip into leftover salt and smear the salt so that sticks on both hands. Take 1/4 of rice mixture from first bowl. Form into a ball. Make it compact, but not so much that the grains of rice become mush. Form into a triangle, square, or cylinder. Repeat for the remaining rice. Each seasoning bowl should yield four rice balls, for a total of twelve. Place each one on a piece of parchment paper to keep them from sticking. Cut each sheet of nori into three strips lengthwise. Then cut each strip in half. Wrap one small strip of nori around the bottom of each rice ball to form an envelope to hold it with. Serve immediately. If you won’t be serving immediately, wait to wrap the rice in the nori, as it will absorb water from the rice and lose its crunch. Instead, wrap tightly and refrigerate. Let the rice balls come to room temperature before serving. Serves 12.
 
YouTube
How to make an Onigiri (rice ball)
imklammerUT
Published on Mar 10, 2010
How to make Onigiri (flavored rice balls)
   
YouTube
How to Make Konbini-Style Onigiri (Japanese Convenience Store Rice Balls) | OCHIKERON
ochikeron
Published on Feb 23, 2016
This video will show you how to make Konbini-style Onigiri (rice balls), individually wrapped in plastic, ensuring that the Nori seaweed sheet stays fresh and crisp.
 
YouTube
What is Onigiri?
Simon and Martina
Published on Jan 27, 2017
What the heck is Onigiri?  Martina is going to give you a small breakdown of Onigiri, what it is, what it’s made from!
     
New York (NY) Times
Rice Balls, Subtle and Showy Alike, at Omusubi Gonbei
By Ligaya Mishan
April 12, 2018
(...)
Its other rice balls, known as omusubi or onigiri in Japanese, are more traditional in ingredients and shape, neatly triangular, trussed with a band of nori or shielded by a shiso leaf like a strategically placed fan. But they, too, are larger than life: Omusubi Gonbei claims to use one and a half times as much rice as its competitors.
   
(This is true both here and at home in Japan. Omusubi Gonbei’s rice balls are giants in both countries, not supersized for American appetites.)
 
The plainest of onigiri is just rice with a press of salt from the fingertips and a stripe of sesame seeds on top. But there’s no just: The rice is grown in Japan without pesticides or chemical fertilizers, then shipped unprocessed and milled only once on the premises, to retain its flavor.
(...)
Omusubi Gonbei
Inside Katagiri Japanese Grocery, 370 Lexington Avenue (entrance on 41st Street between Lexington and Park); 917-472-7168; omusubi-gonbei.com
 
Insider
29 foods you need to try if you visit Japan
Tiana Attride Jan. 16, 2019, 4:31 PM
(...)
Onigiri are bite-sized snacks.
Onigiri are rice balls wrapped in nori seaweed served stuffed with different fillings like salmon pickled plums, and seaweed. Inexpensive and delicious, they are commonly sold in convenience stores.