UFC face pummeler Rampage Jackson got one crazy request from a ballsy fan last night -- dude wanted to get SLAPPED IN THE FACE ... and Rampage obliged.
Rampage was leaving Greystone Manor nightclub in L.A. ... when a fan with a pain fetish approached Jackson and asked to be cracked in the mouth.
Never one to deny a fan ... Jackson bestowed the 5-finger honor across the fan's jaw -- and the fan couldn't have been HAPPIER!
You gotta check out his reaction ... hilarious.
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