Small characters made up on the show South Park. The underpants gnomes are small gnomes that sneak into your house at night to steal your underpants and somehow make a profit out of it.

little gnomes that steal your underwear from drawers and even if you have them on!

A general term for short-ass freshmen (occasionally uperclassmen) who have yet to hit puberty and appear to be in the fourth grade (both in physical appearance, psychological development and writing ability). Usually have some strange/repulsive/childish personal habits (Ex: playing one-man football). Non-anime boobies are like kryptonite to them. Often carry airport bags in lieu of backpacks, believed to be full of stolen underpants (this cannot be confirmed due to their kung-fu grip ability), suspected to be a part of the shadowy and sinister "Phase 2". Reviled by all but the most desperate or demented fanboys and otaku. Sometimes abbreviated to UPGs.

The Underpants Gnomes are mystical creatures that bring you back all of your missing nugs at the most opportune times. Whether you lost your frosty green nuggets during a fumble in the car, or you lost your whole sack during a drunken rampage, the Gnomes are sure to bring back what is rightfully yours in due time. If they happen to give your pot to someone else, do not worry, for the Gnomes are fair and will reimburse you at a later time.
They dwell anywhere that you may have been smoking pot, but these honored creatures never show themselves. Regardless, they must be thanked and honored when they present gifts.