When someone misbehaves, they act badly: they break the rules of how people expect you to act, and they cause trouble or annoy others. We use it most with children — a child who shouts, hits, throws things, or will not listen is misbehaving. Adults can misbehave too, though for serious adult wrongs we often reach for stronger words (cheat, offend, break the law). The word points at the behaviour in that moment, not at the person as a whole, so it is a little softer than calling someone “bad.” Parents, teachers, and school reports use it a lot. The past is misbehaved.