ワンピース
ワンピース · wanpiisu · JLPT N2 · noun
dress
👗 ワン (one) + ピース (piece) = a dress made of one piece, a single garment
Both parts are English loanwords: ワン comes from 'one' and ピース from 'piece'. Japanese often combines English loanwords like this to name clothing and other everyday items.