So called because it grows on old walls and ruined buildings. It
is a native plant. Similarly, wall-cress, wall-creeper, etc.,
are plants which grow on dry, stony places, or on walls. Wall-
fruit is fruit trained against a wall. (See Walnut.) Herrick has
a pretty fancy on the origin of this flower. A fair damsel was
long kept in durance vile from her lover; but at last Up she got
upon a wall, `Tempting down to slide withal; But the silken
twist untied, So she fell, and, bruised, she died. Love, in pity
of the deed, And her loving luckless speed, Turned her to this
plant we call Now the `Flower of the wall.' Young ladies who sit
out against the wall, not having partners during a dance, are
called “wallflowers.” Source: Dictionary of Phrase and Fable, E.
Cobham Brewer, 1894 More on Wallflower from Infoplease: