Originated from the character
Pantalone in the French
Commedia dell'arte who wore distinctive single-piece breeches worn by the character.
Pantalone evolved into the English word Pantaloon, which meant womens' underwear which covered the waist to the knee. Pantaloon eventually evolved into Panties and then Pants for men in England.
Americans however use it as the word for underwear; this actually is because of one of the iconic comic characters -
Superman, who wears his '
pants on the outside' or something to that extent.
Thus, Americans use the word Pants to refer to anything that goes on the outside, and underpants for anything underneath them.
EDIT: Congrats Tsuyu, I was going to point it out to you in case you had not noticed. ^_^