To have or be in a state of extreme emotional fragility or susceptibility, one that is easily influenced or affected by others. Poor Mary has a heart of glass; I don't know how she'll recover from Tom's refusal to go to the prom with her.

A state of extreme emotional fragility or susceptibility, one that is easily influenced or affected by others. Poor Mary has a heart of glass—I don't know how she'll recover from Tom's refusal to go to the prom with her.