English Wikipedia has an article on:

underpants

Wikipedia

From under- +‎ pants.

• IPA^(key): /ˈʌndə(ɹ)ˌpænts/

• Audio (Southern England):

  (file)

underpants pl (plural only, attributive underpant)

1. Underwear covering the genitalia and often buttocks, usually going no higher than the navel.

   Synonym: see at Thesaurus:underwear § Hyponyms

   Hypernyms: underwear; see also Thesaurus:underwear

   Hyponyms: boxer shorts, boxers, briefs, boxer briefs, hipsters, thong, midway briefs, bikini briefs, trunk briefs, trunks; (some are slang) knickers, long johns, tighty whities, gitch, Y-fronts, jocks, butt-huggers

   • 1994, Marketing in Europe, Issues 380-385, page 77:

     The Italian fashion of wearing red underpants on December 31 is becoming popular, reputedly to guarantee luck for the new year.

   • 1999, Thomas Harris, Hannibal, page 19:

     She takes off her fatigue pants and throws them in the washer and the surgical greens and her bloodstained bra and turns on the machine. She is wearing socks and underpants and a .38 Special with a shrouded hammer in an ankle holster.

   • 2011, Les Roberts, The Cleveland Creep: A Milan Jacovich Mystery, page 87:

     He wore khakis slung so low that five inches of his underpants showed—dark blue boxers with big white polka dots.

Despite being a single item of clothing, underpants—like pants, trousers, shorts, &c.—are considered to come in "pairs" and are counted using the phrase "pair(s) of".