To fuck with something. Something you approve of.
Something you like.
Alternatively can also be used to say, i dont fux with you. As in i dont like you
A: Hey, i fux with your fit
(I like your outfit)
B: Thanks man
See you later.
Used in a sarcastic way like you don't plan on seeing that person ever again.
Andy: we're gonna go shoot some heroine Wanna join us?
Jaden: nah fam I gotta go schlatta bois!
Andy: aight, whatever.