monkey business
noun [ U ] uk/ˈmʌŋ.ki ˌbɪz.nɪs/ us/ˈmʌŋ.ki ˌbɪz.nɪs/
骗人把戏;恶作剧
behaviour that is not acceptable or is dishonest
The teacher suspected that there had been some monkey business going on while she was out of the room. 老师怀疑她不在教室里的时候有人搞恶作剧。