Someone told me I was looking wistful. What is wistful?

If you're looking wistful, you are showing or expressing vague yearnings (or longing pensively) for something. In Speak, by Laurie Halse Anderson, Melinda makes an observation about her English teacher, whom she has nicknamed Hairwoman:

Hairwoman tells us they cancelled school for a whole week back in the seventies because of the energy crisis. It was wicked cold and would have cost too much to heat the school. She looks wistful. . . . She blows her nose loudly and pops another smelly green cough drop.