He left for one brief second and he comes back to find the rap game in a shitty state.
All those who took on after him queefed on the rap game. Queef is an expulsion of wind from the vulva during coitus; a vaginal fart (U.D.). He accuses'em of dirtying the rap game and taking it down to ridiculous levels. He could’ve used “shit or fart” to make that point, but since they’re pussies, they queef.
He takes the “dirtying” deeper with the imagery of these rappers even using leaves (in an uncivilized, caveman manner) to wipe their asses after shitting all over. Leaves have a connotation with the texture of the pussy they’re queefing from. He basically said the same exact saying in these two bars.
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He has an allergic reaction to failure that causes him to sneeze when he comes close to it. Causes him to go Atchoo (At you) and achieve. Thus fulfilling/achieving his aims.
You can now also see why he disgusts you, because his allergy to failure causes him to go ATCHOO AT YOU. You better get a handkerchief.
Since his allergic symptoms consist of sneezing, the pause after the ‘a’ of the a-llergic is him about to sneeze (a…), but then he catches himself in time to finish the verse.
Achieve (here pronounced atchee) is also a sneezing noise.
It is also worthy of notice that the song ends on the same note it started with : “Being ill”. This might be interpreted as having been the fittest to survive in the rap competition thanks to his illness (dope rapping) in an illmatic sense.