Or trousers for that matter, if you’re from the UK. I went with pants for the title simply because most of you, dear readers are American. Anyway, the burning question: why are these words so brazenly plural when they clearly refer to a single garment?
The answer is actually fairly straightforward. Trousers is plural partly because of a fairly simple misunderstanding. The word is derived from the old Irish word triubhas, which was singular, and referred to close-fitting shorts. The S at the end led people to assume it was a plural word, thus leading to trousers being plural.
There’s another, much simpler reason that these words, and other related ones such as shorts, jeans, tights, knickers, and drawers are all plural: originally each leg was a separate garment, put on individually, and tied together with a belt or rope. At some point some genius decided to unite the two legs, but the word remained plural anyway.
If it bugs you that we use a plural word for a singular garment by the way, don’t worry. Doubtless hipsters will soon bring individual single-leg trousers back into fashion, and we can strut around the town safe in the knowledge that our vocabulary matches our appearance.