wallflower

Short definition from Wiktionary

Noun

1. 1
   Any of several short-lived herbs or shrubs of the Erysimum genus with bright yellow to red flowers.
2. 2
   Gastrolobium grandiflorum, a poisonous bushy shrub, endemic to Australia.
3. 3
   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.
4. 4
   Any person who is socially awkward, shy, or reserved.

Verb

1. 1
   To stand shyly apart from a dance, waiting to be asked to join in.