1.
If you ~, 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?
PHRASAL VERB: V P, V P with n
2.
If someone fools around with another person, especially when one of them is married, they have a casual sexual relationship.
Never ~ with the clients’ wives...
Her husband was fooling around.
PHRASAL VERB: V P with n, V P
3.
If you ~, 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
PHRASAL VERB: V P