Dictionaries these days define wallflower as a shy or unpopular person not dancing at a party (see Merriam-Webster for instance). Etymonline says the first recorded use of the word in this sense was in 1820, but it restricts it to women:
Colloquial sense of "woman who sits by the wall at parties, often for want of a partner" is first recorded 1820.
The Kipling Society also appears to suggest that in the 1880s wallflower in this sense applied to women only. They explain the use of wall-prop (this is the subject of another question) in Kipling’s “A Friend’s Friend”, Plain Tales from the Hills, 1888:
wall-prop a non-dancer who leans against the wall. (A non-dancing lady was a 'wallflower').
The wall-prop in the story is a man. So, did wallflower in the sense of a non-dancer initially apply to women only? If so when did it start being applied to men as well?
Best Answer
In response to Jacinto's comment, I searched again and found a reference to a man being a wallflower, in which it was not so gender-specific, although in the context it is already known that they are talking about a man. In 1913, Corra Harris wrote In Search of a Husband. On pages 47-48 I found the following passage of conversation between two women:
I've located a couple of other early 20th century references to male wallflowers, which are much more gender-specific.
From 1910, in At the home plate, by Albertus T. Dudley, page 123:
From March 1918, in The Recruit. A pictorial naval magazine. v.4 no.3 at the end of the first paragraph of page 24, from a short story called "The Wall Flower":
Further back in 1884/5, in The Freemason's repository. v.14 (following a list of questions regarding active participation in the Freemasonry), on page 133: