Literally, it means to have one's pants pulled down and left in one's undies, or nothing at all.<br />
Metaphorically, it means some sort of ignominious defeat.
its what kids are calling 'flagged' these days
it means your trousers are pulled down
when i was in school it was when someone ran up to a guy and yanked his pants to his ankles sometimes they was lucky enough to still have underwear on