Underwear is the clothes worn next to the skin. Clean underwear, thermal underwear, a drawer of underwear.
It is uncountable, so a pair of pants rather than an underwear. Thermal underwear is the base layer worn for warmth. The word is a nineteenth-century euphemism replacing a set of blunter ones, which is why it sounds so neutral. Note underclothes and smalls, both British; and that Dutch ondergoed and German Unterwäsche are built the same way.
1noununderwearClothes worn next to the skin.
“Pack enough clean underwear and socks for the whole week.”
underwear is clothing.
Under plus wear, a nineteenth-century formation. Dutch ondergoed, German Unterwäsche and Swedish underkläder are built from exactly the same parts.
Middle English
weren
English
wear
English, today
underwear
From under- + -wear. From under + wear. Compare Dutch ondergoed, German Unterwäsche, Swedish underkläder. wear comes from Middle English weren. Middle English weren, werien, from Old English werian (“to clothe, cover over; put on, wear, use; stock (land)”), from Proto-West Germanic waʀjan, from Proto-Germanic wazjaną (“to clothe”), from Proto-Indo-European wes- (“to dress, put on (clothes)”). Cognate to Sanskrit वस्ते, Ancient Greek ἕννυμι (“put on”), Latin vestis (“garment”) (English vest), Albanian vesh (“dress up, wear”), txb wäs-, xcl զգենում, Welsh gwisgo, hit 𒉿𒀸-.
Etymology from Wiktionary, CC BY-SA