Can "wallflower" be used as an idiom? If so, what does it mean?

[rita]

Test No. incompl/inter-15 "Popular Idioms (2) Garden", question 4

Poor Mary was a bit of a ......... at the dance.

(a) cornflower
(b) cauliflower
(c) wallflower
(d) wildflower

Test No. incompl/inter-15 "Popular Idioms (2) Garden", answer 4

Poor Mary was a bit of a wallflower at the dance.

Correct answer: (c) wallflower

Your answer was: incorrect
Poor Mary was a bit of a cauliflower at the dance.
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Hi Alan,

could you tell me the meaning of wallflower
thanks

[Alan]

This refers to a girl who is not being noticed/being ignored. The flower itself usually grows away from other plants against a wall.
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English as a Second Language
You can read my ESL story Saying It Twice

[Guest]

[Helene]

[Kathryn Wong]

Dear Alan,

Can i say " Mary was a bit of a wallflower at her look?"

[Alan]

Hi Kathryn,

I'm afraid your sentence: