To misunderstand is to fail to understand someone or something correctly. You do have an idea in your head — but it is the wrong idea. That is different from not understanding at all, which is just a blank. You can misunderstand a word, a joke, a look, an email, or a whole person. People often say “Don’t misunderstand me” when the next sentence could sound harsher than they mean. The mix-up can be small (“I thought you said Tuesday”) or serious (“They thought I didn’t care”). Past forms: misunderstood. The noun is misunderstanding.