cheat on someone
phrasal verb with cheat verb [ I or T ] uk/tʃiːt/ us/tʃiːt/
informal
B2 对（丈夫或妻子）不忠
If you cheat on your husband, wife, or usual sexual partner, you secretly have a sexual relationship with someone else.
She found out that he'd been cheating on her. 她发现他对她不忠。