late 14c., agent noun from knock (v.). Sense of "door banger" is by 1590s. Knockers "a woman's breasts" is slang attested from 1941.
Entries linking to knocker
knock v.
Old English cnocian (West Saxon cnucian), "to pound, beat; knock (on a door)," likely of imitative origin. Figurative meaning "deprecate, put down" is from 1892. Related: Knocked; knocking. Of engines from 1869. To knock back (a drink) "swallow quickly or at a gulp" is from 1931. Many phrases are in reference to the auctioneer's hammer, for example knock down (v.) "dispose of (something) at auction" (1760).
door-knocker n.
"metal device fixed to the outside of a door for banging to give notice when someone desires admission," 1794, from door + knocker (n.).