Undershorts are men's or boys' underpants — the short garment worn under trousers. Like trousers and shorts themselves, the word is always plural in shape: a pair of undershorts, my undershorts are in the wash. You cannot have 'an undershort'. It is chiefly American, and it feels slightly formal, careful, or old-fashioned — the word a clothing catalogue or a school packing list would use. In real speech Americans say underwear, boxers, or briefs; British speakers say pants or boxers. So learn undershorts to recognise it, and use 'underwear' when you speak. Note also the trap in 'pants': in the US, pants means trousers, so "he was in his pants" is a normal sentence there and a funny one in Britain, where pants means underwear.

男用內褲。台灣就說「內褲」；平常英文說 underwear 或 boxers 更自然。