Silvermasking is a fraternity sex game played on girls by frat guys. One frat brother begins having sex with a girl in private. In the middle of intercourse, other frat brothers burst into the room wearing nothing but masks made of tinfoil and tinfoil covering their genitals. The silvermasked frat brothers dance around the copulating couple, typically having the effect of seriously freaking out the girl. The reaction of the frat brother who is engaged in intercourse could range from embarrassment, to anger, to unabashed hilarity -- mostly depending on whether he was expecting the prank or not. Silvermasking is purported to have been a common prank (maybe invented?) at the SAE fraternity at the University of Illinois in the mid-1990's.
"Hey, I hear you hooked up with that Chi-O hottie last night!"
"Yeah, but those senior bastards silvermasked me in the middle of it. I mean, it *was* really damn funny, but I am kind of pissed that the interrupted an ruined one of my most epic hook-ups ever."