Very simply, a hat for your bum.
Dan was walking down the road when he noticed his bum was getting sun burnt. At this point Dan wished he had a bumhat.
by PMizzle July 12, 2005
The act of putting asomeone's exposed butt close to someone else's face who has passed out at a party and taking a picture or video of the act.
We gave John a bumhat last night.
by boothash October 16, 2004