Unbecoming describes behavior or appearance that is inappropriate or unsuitable, especially when it clashes with what's expected of someone. When your teacher says that shouting in the hallway is unbecoming of fifth graders, she means you're acting beneath the maturity level you should have reached. When someone says a wrinkled, stained shirt is unbecoming for a formal dinner, they mean it doesn't fit the occasion.

The word carries a sense of disappointment: not that something is merely wrong, but that it falls short of a standard. A captain who panics during a crisis shows behavior unbecoming an officer because leaders are expected to stay calm under pressure. Gossiping about a friend can be described as unbecoming behavior because it violates the trust and loyalty that friendship requires.

Notice how unbecoming often appears with “of”: behavior unbecoming of a champion, conduct unbecoming of a student. This construction emphasizes the gap between what someone does and what their role demands. The word suggests that someone is capable of better and should know it. When applied to clothing or appearance, it simply means unflattering or unsuitable, though this usage is somewhat old-fashioned today.