Wanna see grown-ass college football players cower in fear over a harmless Halloween prank??

Good ... 'cause University of Pittsburgh football staffers scared the crap out of their team with a fake mannequin -- and the video of the players' reactions is HILARIOUS!!

The team's assistant director of player personnel, Karlo Zovko, pretended he was a dummy wearing a Pitt uniform ... and when guys walked by, he pounced.