If you fool around, you behave in a silly, dangerous, or irresponsible way. They were fooling around on an Army firing range Have you been fooling around with something you shouldn't?
indulge in horseplay; "Enough horsing around--let's get back to work!"; "The bored children were fooling about"
If someone fools around with another person, especially when one of them is married, they have a casual sexual relationship. Never fool around with the clients' wives Her husband was fooling around
If you fool around, you behave in a playful, childish, and silly way, often in order to make people laugh. In British English, you can also say you fool about. They fooled around for the camera. = mess about