His head up his ass, I'mma have to head-butt him

from Lil Wayne – Goulish (Pusha T Diss) Lyrics on Rap Genius

Meaning

Maybe Wayne should have articulated his head/ass and head-butt wordplay a bit differently. He was probably going for Pusha figuratively thinking he is the shit, but only because his head is in his ass. Then, to utilize the head/ass/butt wordplay, he inserts a way to suggest he would fight Pusha.

But then there is this interpretation from our friend BigGhostFase: https://twitter.com/bigghostfase/status/206253753609883648

To help improve the meaning of these lyrics, visit “Goulish (Pusha T Diss)” by Lil Wayne Lyrics and leave a comment on the lyrics box