c.1200, a double plural, from Old English brec "breeches," which already was plural of broc "garment for the legs and trunk," from Proto-Germanic *brokiz (cf. Old Norse brok, Dutch broek, Danish brog, Old High German bruoh, German Bruch, obsolete since 18c. except in Swiss dialect), perhaps from PIE root *bhreg- (see break (v.)). The Proto-Germanic word is a parallel form to Celtic *bracca, source (via Gaulish) of Latin braca (cf. French braies), and some propose that the Germanic word group is borrowed from Gallo-Latin, others that the Celtic was from Germanic.
Expanded sense of "part of the body covered by breeches, posterior" led to senses in childbirthing (1670s) and gunnery ("the part of a firearm behind the bore," 1570s). As the popular word for "trousers" in English, displaced in U.S. c.1840 by pants. The Breeches Bible (Geneva Bible of 1560) so called on account of rendition of Gen. iii:7 (already in Wyclif) "They sewed figge leaues together, and made themselues breeches."