Yeah, I do away with nine niggas in nine days
My nine sprays when shit don't go my way
Hold it sideways, fuck around, cause a crime wave

from 50 Cent – Crime Wave Lyrics on Rap Genius

Meaning

50 will kill you simply cause the end results weren’t as he had envisioned it — capping one guy by each day!

Holding a gun sideways actually doesn’t have any practical benefits and could even make proper aiming more difficult. However, due to its portrayal in American movies the side grip has become quite popular in the hip hop culture and among street criminals. It was popularized through the opening scene in Menace 2 Society, a sure favorite of any gangster rapper. Hey, at it looks cool!

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