From Yahoo Answers --
We've heard it's because pants and shorts have two legs. But shirts have two sleeves, and you don't have a "pair of shirts." Hmm, so much for that theory.
According to The Mavens' Word of the Day, pants in the plural form is an Americanism first recorded in 1840. The word is short for pantaloons, a term that originated with a character in Italian commedia dell'arte who wore both stockings and breeches.
World Wide Words states that these types of clothing (pants, underwear, shorts, tights -- or the equivalent terms for them) were made in two parts. One part for each leg, then belted in the middle, somewhat like chaps on cowboys. Over time, they ended up as one piece of clothing, but the habit of referring to the old "pair" persisted.
This reference isn't universal, however. The clothing industry often uses the singular form of pant, but not consistently. So pick your favorite, singular or plural -- you'll still put them on one leg at a time.
hwell:
