He likes his Americanism's
More like I just cannot be bothered with being pedantic...
Still, since you're going to push the point... "pants" (the word and its association) came from the 19th century
British term "pantaloons", with its roots coming from the 3rd century canon "Saint Pantaleone".
Pan being greek for "all",
leo is the Latin word for "lion". Eventually he became the patron saint of physicians, and through his canonization for "courage" and "strength" (having survived six execution attempts at the hands of the Romans), many boys were christened in his name for centuries.
"Pants" came into the vernacular through a comic drama as a character called "Panteleon" ("all lion") being synonymous with "the comic fool". The actor playing this character on stage would wear breeches (another British word, by the way) that were tight below the knee, but bloused out between the waist and the knee.
Weirdly, in the 18th century this became the fashion (for men) throughout western Europe (and Great Britain), and so leg-covering clothing became known as "pants". Since clothing worn against the skin at this point was already known as "under garments", the logical distinction of "underpants" became synonymous with any lower garment worn under the pants.
And so it is not an American term at all, but in fact British by way of Roman (Latin) and Greek origin.
So, now that I have indulged in the pedantic, perhaps we can dispense with the linguistic pissing contests for a while; what do you say?
EDIT: Also, the word "pantaloons" became disassociated with men's clothing completely, and became associated with (eventually defined as) a woman's undergarment to be worn under the hooped skirts of the same period.
EDIT AGAIN: Just for the record, I was made to study Latin at grammar school... so if I use a word you find "strange" you should perhaps consider that all people are different, rather than expecting everyone to conform to
your particular mannerisms. It comes across as offensive to criticise someone for using a perfectly legitimate word just because you are unaware of its national origins.