Someone is not getting an obvious joke or is acting strange
Jit - Short for legit (meme culture turned git into jit
Trippin’ - strange behavior, acting/saying things that suggest that you are gay
jit means that guy or dude or bro. And Trippin mean you said or did something crazy. Nah in this sentence mans in awe. Combine the words, and you get nah jit Trippin