To be hit or slapped unexpectedly by another person. To be "clowned" the receipent of the assault must be in full view of others, and the contact must be limited to one single action that produces; a flailing motion, reeling motion, or any action of retreat on the assaultee's part that appears humourous to onlookers.
"So we were at the club and this guy is mackin' on someone's girlfriend. And the boyfriend is right behind him, but the guy is too drunk to notice. The boyfriend just reached out and clowned him right upside the head."