play around

informal
(UK also play about)
to behave in a silly way:
Stop playing around and get on with your homework!
I wish you wouldn't play about with that - you'll break it.

informal disapproving
(UK also play about)
If someone who is married or has a serious relationship plays around, they have sex with another person or people:
If she finds out he's been playing around with his secretary, there'll be trouble.
(Cambridge Advanced Learner's Dictionary & Thesaurus からの play around の定義 © Cambridge University Press)

アメリカ英語辞典
to have a sexual relationship with someone other than your husband or wife
(Cambridge Academic Content Dictionary からの play around の定義 © Cambridge University Press)

- Cambridge Dictionary