So all ya new jacks kickin' wack raps
It's a fact that I'll be on your fuckin' back like a knapsack
from Big L (Ft. Jay-Z & Lord Finesse) – Da Graveyard Lyrics on Rap Genius
Meaning
New rappers without appreciation for the art of hip hop (new jacks) need to watch their back when Finesse is around because “it’s a fact” that Finesse—a respected rapper true to the form—will attack them. He emphasizes the child-like abilities of the new jacks by referencing napsacks

Crazy double syllable wordplay here
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