means A shy or socially overlooked person who lingers at the edges of a party or gathering rather than joining in.

from From the literal plant — the wallflower (genus Erysimum), a fragrant bloom that takes root in old walls and rocky crevices, clinging to the stone at the side of things. By the early 19th century the name had bloomed into a metaphor for the person pressed against the wall at a dance, present but unpicked, decorating the room's edges much as the flower decorates a crumbling wall.