He admitted he had cheated on his wife with a colleague.

cheated 是动词 cheat 的过去式，cheat 的意思是"欺骗、作弊"。 on 在这里是介词，表示"关于、针对"，与 cheat 搭配使用时表示"在某方面欺骗某人"。 cheat on 是一个固定短语，特指"在感情或婚姻关系中不忠、背叛伴侣"。