mess around with somebody
1. to treat someone badly He was always messing around with her, and she was right to leave him.
2. to have sex with someone other than your husband, wife, or usual sexual partner fool around with somebody She found out that her husband was messing around with her best friend.
mess around with something
to amuse yourself by doing or saying something that is likely to cause trouble fool around with something I don't know if I'm dealing with a 14-year-old messing around with a computer or if I'm dealing with organized crime.