A sharp-tongued person says things that cut. The words may be clever, but they are not kind. They point out what is wrong, or they mock, and people feel hurt. A sharp-tongued aunt might love you and still leave you small after dinner. Sharp here is about the tongue as the place speech comes from — not about thinking fast. That other word is sharp-witted. We usually say sharp-tongued when the speaking feels unfriendly, even if we half-admire how quick the remark was. You will hear it in stories and in talk about someone's character more than in very careful, official writing.