Unauthorized Ass Contact. Occurs when two people try to slide past each other in a narrow space and physical contact accidentally occurs below the belt. Normally this is a non-issue between strangers in a crowded place, but calling out the UAC among co-workers lets the other person know it was an accident and that you're not trying to be gay. Originally coined for use by missile combat crewmembers in Minuteman III launch control centers.
*brush* Sorry about the UAC. *grunt*