To mess with; to interact with in a careless or inappropriate way.
Don't fuck with that gang! They all carry guns!
To play with in a lighthearted or joking manner; or kid with
Don't take it serious, man, I'm just fucking with you.
To affect seriously or disturbingly; to trouble
The thought that she might be cheating on me is really fucking with me.
To work or associate with, roll with, have dealings with; (by extension) to like, approve of, enjoy.
I fuck with you heavy.