Closer than mustache with top lip
I must stash the pump six

from Lil B – Ridin' 4 My Niggaz Lyrics on Rap Genius

Meaning

A mustache and a top lip are pretty damn close. “Must stash” is a play on “mustache.” He’s saying he has to hide his gun.
He could be saying the he is as close to his gun as a mustache is to a top lip.

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