If you say that someone has misjudged a person or situation, you mean that they have formed an incorrect idea or opinion about them, and often that they have made a wrong decision as a result of this. Perhaps I had misjudged him, and he was not so predictable after all
mis·judg·ment misjudgments in BRIT, also use misjudgement A misjudgment is an incorrect idea or opinion that is formed about someone or something, especially when a wrong decision is made as a result of this. a misjudgment in British foreign policy which had far-reaching consequences Many accidents were due to pilot misjudgement