Underpants are the garment covering the hips and bottom, worn beneath trousers or a skirt. Always plural, like everything else you put both legs into.
British English uses pants for the same thing, which is the trap for Americans, since American pants are trousers and the confusion is a permanent source of comedy. Knickers is the usual British word for the women's version. Boxers, briefs and trunks name the styles. The whole category is underwear, which covers rather more.
1noununderpantsAn undergarment covering the lower body.
“He was down to a pair of underpants and a T-shirt.”
From under- + pants.
Etymology from Wiktionary, CC BY-SA