breeches: [OE] The theoretical singular of this word, breech, comes from a form which in Old English was plural – brēc. Its unrecorded singular, which would have been *brōc, came from a prehistoric West and North Germanic *brōks. The word’s ultimate origin is not known, although some connect it with break; and it is possible that it was borrowed early on into Gaulish as brāca, the probable source of English bracket. The Old Norse descendant of the Germanic form, brók, was not only partly responsible for the Scottish version of breeches, breeks, but is also the source of brogue. => brogue
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 (cognates: 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 (aource of 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."