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cheat on someone
to commit adultery; to be unfaithful to one's spouse or lover. "Have you been cheating on me?" cried Mrs. Franklin. He was caught cheating on his wife.
cheat on somebody
to have a sexual relationship with a person while you are married to someone else or are having a relationship with someone else It's hard to believe that he cheated on you.
Be sexually unfaithful to, as in They broke up right after she found he was cheating on her. [Colloquial; 1920s]
1. To behave fraudulently during some process or activity: The teacher caught the student cheating on the test.
2. To be unfaithful to someone, especially a spouse or lover: I hired a private detective to see if my spouse was cheating on me.