Term used often in northern states, it's the action of rubbing a handful of snow in someone elses face, hard. Whitewashes can also be done by shoving their head into the ground and making them eat snow. Usually it's a significant end of a snowball fight.
I was kicking his ass in the snowball fight, so I just whitewashed him and left.
or, before a fight--
Dude, no whitewashes, those are gay. Your gloves are like fuckin sandpaper.