trousers (n.)

c. 1200, trussen, "to load, load up, pack up in a bundle," from Anglo-French trusser, Old French trusser, torser "to load, fill, pack, fasten" (11c.), from Old French trousse, torse "package, bundle" (see truss (n.)).

Specifically "to draw close the garments and laces" (of someone), mid-15c., hence a general sense of "to dress," as in trussed up "dressed" (it also could mean "hanged"), from 1530s. Related: Trussed; trussing; trussage "baggage" (c. 1500), from Old French troussage, variant of torsage.

"trousers, drawers," 1840, see pantaloons. The word was limited to vulgar and commercial use at first.

I leave the broadcloth,—coats and all the rest,—

The dangerous waistcoat, called by cockneys "vest,"

The things named "pants" in certain documents,

A word not made for gentlemen, but "gents";

[Oliver Wendell Holmes, "Urania: A Rhymed Lesson," 1846]

Colloquial singular pant is attested from 1893. To wear the pants "be the dominant member of a household" is by 1931, also with trousers. To do something by the seat of (one's) pants "by human instinct" is from 1942, originally of pilots, perhaps with some notion of being able to sense the condition and situation of the plane by engine vibrations, etc. To be caught with (one's) pants down "discovered in an embarrassing condition" is from 1932.