A joking word for underwear. It comes from a time when polite people would not name such clothes at all, so they called them 'the things one does not mention' — and English kept the joke long after the shyness faded. Today nobody uses it seriously; it is what an aunt says while hanging out the washing, or what a comedian says to raise a smile. The humour depends entirely on the mismatch: a very long, very grand-sounding word for something very ordinary. It is always plural, and usually has 'my' or 'her' in front of it. Worth knowing that the word originally meant trousers, not underwear — in the 1820s even trousers were too indelicate to name — and it drifted downwards to underwear about a hundred years later, as the boundary of what could be said politely moved. That drift is the whole history of the word in one line. You will also hear it stretched to any other embarrassing topic mentioned in the plural, but underwear is the standard meaning.

內衣褲（詼諧、老派的說法，類似「貼身衣物」帶開玩笑的口吻）