“Whatchamacallit” (what you may call it) is the name of a candy bar introduced in 1978 by The Hershey Company. The slang name is of unknown authorship.
   
“Whatchamacallit” was printed in The High School World (Topeka, KS) on March 31, 1910. “Whachamacallit” was printed in the Hampton-Columbian Magazine (New York, NY) on January 1912. “Watchamacallits” was printed in the Washington (DC) Times on March 6, 1914.
 
   
Wiktionary: whatchamacallit
Etymology
A shortened version of “what you may call it” or “what you might call it”. United States, 20th century (recorded in 1921).
Noun
whatchamacallit (plural whatchamacallits)
1. (slang) A metasyntactic term used for any object whose actual name the speaker does not know or cannot remember; a doodad, gizmo, thingamajig, thingy.
   
Wikipedia: Whatchamacallit (candy)
Whatchamacallit is a candy bar marketed in the United States by The Hershey Company.
 
History
Whatchamacallit bars were first introduced in 1978. The name was devised by Sallie Grayson, the writer of STUFFED: Adventures of a Restaurant Family when she was the Associate Creative Director at Doyle Dane & Bernbach and was in charge of new brands on the Hershey account. From 1978 to 1987, Whatchamacallit consisted of a bar of peanut-flavored crisp that utilized peanut butter as the flavoring agent, coated in a thin layer of chocolate. From 1987 to 2008, Whatchamacallit has included peanut-flavored crisp that utilizes peanut butter as the flavoring agent, with a layer of caramel and a layer of chocolate coating.
         
(Oxford English Dictionary)
whatchamacallit, n.
Etymology: Representing a pronunciation of what-you-may-call-it (see what-ye (or -you) -call(-it) at what-d’ye-call-‘em n. γ. )
Chiefly U.S.
= what-d’ye-call-‘em n.
[1928   M. Ostenso Mad Carews xii. 160   It’s your—whatcha-may-call-it—your dowry!]
1942   L. V. Berrey & M. Van den Bark Amer. Thes. Slang §75/4   Contrivance..gadget, whatchamacallit.
 
Newspapers.com
31 March 1910, The High School World (Topeka, KS), “The Play Last Night,” pg. 4:
Skinny Wilcuts and Phil Gray nearly broke their backs bearing the latter in an improvised Japanese Whatchamacallit.
   
January 1912, Hampton-Columbian Magazine (New York, NY), “The Ladder of Consequences” by Harris Merton Lyon, pg. 834, col. 1:
“Kummel with rum float,” said Archibald Cuteau Gruntt, the Second Assistant Whachamacallit, Department if Justice.
   
Newspapers.com
30 October 1912, “Odd Fellows in Bazaar Surprises,” pg. 2, col. 3:
... a “funny girl-eating whatchamacallit out of the Blue Danube,” ...
 
Newspapers.com
6 March 1914, Washington (DC) Times, “Skillets and Cymbals Models for New Hats,” pg. 11, col. 7:
Jumping with great gusto into the pun column, it may be said that the lid was taken off for society women at the Ritz-Carlton today, where the collection of Paquin gowns and other feminine watchamacallits are being exhibited.
 
Newspapers.com
19 July 1920, Dixon (IL) Evening Telegraph, pg. 7, col. 7:
WHACHAMACALLIT IS PUZZLE TO SCIENTISTS
     
OCLC WorldCat record
What’s what : the whatchamacallit book.
Author: Hammond Incorporated.
Publisher: Maplewood, N.J. : Hammond, ©1981.
Edition/Format:   Image : Picture : English
Summary:
Shows the names of the parts of a hardbound and softbound book.
   
OCLC WorldCat record
The great whidgit whatchamacallit and thingamajig idea book
Author: Sue Thomas; Betsy Warren; Dave Ferguson; Tom Warren; Boy Scouts of America. St. Louis Area Council.
Publisher: St. Louis, Mo. : St. Louis Area Council, Boy Scouts of America, 1987, [©1983]
Edition/Format:   Print book : English : 2nd ed
Summary:
A handbook intended for Cub Scout leaders which provides ideas and detailed plans for ceremonies, banquets, and meetings. Includes clip art, crafts, games, skits and more.
 
(Trademark)
Word Mark WHATCHAMACALLIT
Goods and Services IC 030. US 046. G & S: CONFECTIONERY. FIRST USE: 19780911. FIRST USE IN COMMERCE: 19780911
Mark Drawing Code (5) WORDS, LETTERS, AND/OR NUMBERS IN STYLIZED FORM
Serial Number 73186501
Filing Date September 21, 1978
Current Basis 1A
Original Filing Basis 1A
Registration Number 1130075
Registration Date January 29, 1980
Owner (REGISTRANT) HERSHEY FOODS CORPORATION CORPORATION DELAWARE 19 E. CHOCOLATE AVE. HERSHEY PENNSYLVANIA 17033
(LAST LISTED OWNER) HERSHEY CHOCOLATE & CONFECTIONERY CORPORATION CORPORATION BY MERGER, BY ASSIGNMENT DELAWARE 4860 Robb Street Suite 204 WHEAT RIDGE COLORADO 80033
Assignment Recorded ASSIGNMENT RECORDED
Type of Mark TRADEMARK
Register PRINCIPAL
Affidavit Text SECT 15. SECT 8 (6-YR). SECTION 8(10-YR) 20090812.
Renewal 2ND RENEWAL 20090812
Live/Dead Indicator LIVE