the word 'underpants' is a transparent english compound formed from the adverb 'under' and the noun 'pants'. 'pants' itself is a clipping of 'pantaloons', which entered english via french and italian, named after the venetian comic character pantalone, stereotypically depicted wearing tight trousers. the compound logically denotes garments worn beneath the outer trousers. its formation follows a common english pattern where 'under-' prefixed to an item of clothing specifies its position as a lower or inner layer.

Structure: under (beneath) + pants (trousers) "under" (old english, preposition/adverb meaning 'beneath, below'); "pants" (shortened from 'pantaloons', from italian 'pantalone', ultimately from saint 'pantaloun', a character in comedy who wore tight trousers, functional role: core noun).