www.alphadictionary.com

Pants is a curious word: It looks like it is plural, but it isn't; it names a single object. Its meaning is "trousers," or "breeches," both words that have the same characteristic. One does not put on a pant or a trouser or a breech, but pants, trousers, or breeches. However, one leg is called a pant, as in "He was repairing a hole in the pant leg."

The ultimate origin of pants (filtered through Italian and French) is "pantaloon," which comes from a Venetian character in 16th century Italian commedia dell'arte represented as a foolish old man wearing pantaloons, baggy trousers fastened at the ankles.

But where did the word pants get its plural? And why can't one make it a plural in the usual way: "pantses" (sounds like Gollum)? Breeches with connected legs at the top have been made for centuries, so it's not as if they were a new and wonderful thing in 16th century Italy.

The inquiring mind of Mike Nichols wants to know!

For what it's worth, here's my theory: Pantaloon was the character, the things he wore were his, so were called pantaloon's. Subsequently shortened to pants.