mess around
mess around {v. phr.}
1. To engage in idle or purposeless activity.
Come on, you guys, — start doing some work, don't just mess around all day!
2. {vulgar}
To be promiscuous; to indulge in sex with little discrimination as to who the partner is.
Allen needs straightening out; he's been messing around with the whole female population of his class.
Compare: FOOL AROUND.
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Source: A Dictionary of American Idioms