To be frumpy is to be out of date style-wise. If you want to get on your mom's bad side, tell her she looks frumpy.
Have you ever seen one of those old one-piece bathing suits that cover almost the entire body? A woman wearing one of those today would be considered frumpy. This word has to do with how people — especially women — dress. A fashion model is not likely to be frumpy. When you look frumpy, your clothes are out of date and also modest, boring, or drab. Frumpy is the opposite of stylish.