wallflower

noun

person who attends social gatherings but does not participate, especially due to shyness

Pronunciation: /ˈwɔːlˌflaʊ.ə/ / /ˈwɔlˌflaʊ.ɚ/ / /ˈwɑlˌflaʊ.ɚ/

Etymology: From wall + flower.

Any of several short-lived herbs or shrubs of the Erysimum genus with bright yellow to red flowers.
“A polypetalous corolla is either cruciform, as in a wall-flower, rosaceous, papilionaceous, as in the pea kind, or incomplete, when some parts found in analogous flowers are wanting.”
Gastrolobium grandiflorum, a poisonous bushy shrub, endemic to Australia.
A person who does not dance at a party, due to shyness or unpopularity; by extension, anyone who is left on the sidelines while an activity takes place.
“Mrs. Galbraith shook all over with laughter as she replied, "Hear that boy, asking me to dance ! I'm content to be a wallflower, now-a-days."”
“And now, by virtue of his office, he is entitled to a seat in the Grand Lodge. Is it any wonder he is a wall-flower there […]”
Any person who is socially awkward, shy, or reserved.
“I've always been a wallflower, even in my own home. But, I'm willing to learn to be a part of your world. I would like to. I have already told my parents that I want to go to soirées.”

Etymology: From wall + flower.

To stand shyly apart from a dance, waiting to be asked to join in.
“[…] the idea that a full tango experience is impossible without the presence of wallflowers and without the threat of wallflowering as the potential dancers enter the tango club.”