breech (n.)

"back part of a gun or firearm," 1570s, from singular of breeches (q.v.) in the sense "lower part of the body," hence "the hinder part of anything" (especially the part of a cannon or firearm behind the barrel). Breech-loader is from 1858.

late 15c., "put in breeches," from breeches. Meaning "fit a gun with a breech" is from 1757, from breech (n.). Related: Breeched; breeching.