A “plain Jane” is a girl or a woman who is of average or unremarkable appearance. The person may or may not actually be named “Jane.”
   
“Plain Jane” was printed in The Southern Literary Messenger (Richmond, VA) in January 1859. The story “Plain Jane” by Mattie Dyer Britts was printed in The Hancock Democrat (Greenfield, IN) on February 1, 1872, as well as in other newspapers on other dates.
   
 
Wiktionary: plain Jane
Noun
plain Jane (plural plain Janes)
1. (informal) A young woman or girl of average or unremarkable appearance.
 
(Oxford English Dictionary)
Plain Jane  n. and adj. (also plain Jane)  (a) n. a girl or woman who is unattractive or simple in appearance or dress; (hence) an object, etc., which is basic, not ornamental, unattractive, etc.;  (b) adj. unadorned, simple, ordinary, undistinguished.
1912   C. Mackenzie Carnival ii. 14   She sha’n't be a Plain Jane and No Nonsense, with her hair screwed back like a broom, but she shall be Jenny, sweet and handsome, with lips made for kissing and eyes that will sparkle and shine.
1936   C. Day Lewis Friendly Tree i. 11   It was the plain-Jane, methodical part of herself.
     
January 1859, The Southern Literary Messenger (Richmond, VA), “Letter II” by J.D.P., pg. 61, col. 1:
As to compare small things with great, fanciful mothers, whose names like yours and mine, are plain Jane or Mary, or it may be Deborah or Judith, ...
     
7 September 1868, Boston (MA) Daily Advertiser, “The Newport Season,” pg. 1, col. 4:
There she sits, fast and free, beautiful or not, as the case may happen—Di Vernon, or plain Jane Potiphar.
 
30 July 1869, Jamestown (NY) Journal, pg. 8, col. 5:
She isn’t Martin’s wife at all, but plain Jane Rogarden.
   
Newspapers.com
1 February 1872, The Hancock Democrat (Greenfield, IN), pg. 1, col. 4:
Select Story.
PLAIN JANE.
BY MATTIE DYER BRITTS.
(...)
They were all beauties, and good, kind girls in their way, but I was only Plain Jane.
 
Newspapers.com
22 February 1872, Kansas Reporter (Louisville, KS), pg. 1, col. 2:
Selected Story.
PLAIN JANE.
BY MATTIE DYER BRITTS.
(...)
They were all beauties, and good, kind girls in their way, but I was only Plain Jane.
 
Newspapers.com
4 August 1877, Fresno (CA) Republican, “Women of the Orient,” pg. 1?, col. 7:
... so that in place of their musical Hindostanee names they are called plain Jane Smith, Sarah Brown, or something of that sort.
 
OCLC WorldCat record
My little plain Jane. Song, words and music by G. Trevenor.
Author: George Trevenor
Publisher: New York : W.A. Pond, 1895.
Edition/Format:   Musical score
 
OCLC WorldCat record
Just plain Jane. [Song.] Words by Geo. Totten Smith.
Author: Robert A Keiser
Publisher: New York : Leo Feist, [1903]
Edition/Format:   Musical score
 
OCLC WorldCat record
Just plain Jane. [Song.] Words by Harry A. Bloomberg.
Author: Alfred H Bloomberg
Publisher: [New York] : M. Witmark & Sons, [1903]
Edition/Format:   Musical score
 
OCLC WorldCat record
Margery Morris and plain Jane
Author: Violet Gordon Gray
Publisher: Philadelphia : Penn Pub. Co., 1920.
Edition/Format:   Print book : Fiction : Juvenile audience : English
 
OCLC WorldCat record
Don’t take your troubles to bed : Louis I. Squith and Walter Brooks present “Plain Jane” a captivating musical comedy in 2 acts
Author: Tom Johnstone; Phil Cook, (Composer); John Arpin Collection - York University.
Publisher: New York : Harms Inc, 1924.
Edition/Format:   Musical score : English
 
Urban Dictionary
plain jane
Ordinary, average female, moderately attractive but mentally stable.
I’ll take a plain jane over a hot psycho-bitch every time, man.
#babes#women#girls#average#normal#ordinary #plain#nofrills#stable#ok#notbad#acceptable#standard issue
by John Ronald November 19, 2007
 
Urban Dictionary
Plain Jane
A watch, typically a Rolex, without any diamonds added to it. See the song “Plain Jane” by A$AP Ferg
Why would you lower the value of your Presidential? Skip the diamonds; keep it plain jane.
by Forest Blump September 13, 2017