Hit 'em with the Nina man
Or that 4/5th guaranteed to lean ya man

from Birdman (Ft. Clipse) – What Happened to That Boy? Lyrics on Rap Genius

Meaning

Either the “Nina” (9mm) or the .45 caliber pistol will kill you

He’s using “lean” in a non-standard way; when a rapper talks about “leaning” he generally means getting high on promethazine and cough syrup (which will, eventually, kill you)

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