I might crack a smile but ain't a damn thing funny

from Mobb Deep (Ft. Nas & Raekwon) – Eye for an Eye Lyrics on Rap Genius

Meaning

A classic tough-guy move is to chuckle while muscling someone, and then get angry when your victim laughs along with you:

http://youtu.be/hTIYIqUwhkM?t=2m31s

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