fool around
US /fuːl əˈraʊnd/
UK /fuːl əˈraʊnd/

1.
to behave in a silly or playful way, often wasting time
:
•
Stop fooling around and get your work done.
•
The kids were fooling around in the backyard.
2.
to have a casual sexual relationship with someone
:
•
He was accused of fooling around with a colleague.
•
They were just fooling around, nothing serious.