If a woman looks frumpy, people mean her clothes look old, dull, and out of date, so she does not look attractive in a fashionable way. The word can also describe the clothes themselves (a frumpy coat) or the whole look. It is informal, and it is unkind if you aim it at a person. Though actually, fashion keeps moving, so frumpy is often other people's taste talking, not a hard fact. Men can be called frumpy too, but the word grew up pointing at women.