knockoff (n.)

also knock-off, "cheap imitation," 1966, from the verbal phrase knock off "do hastily" (1817), in reference to the casual way the things are made. The verbal phrase knock off is attested from 1640s as "desist, stop" (work, study, etc.), hence knockoff (n.) "act of leaving work" (1899) and, probably, the command knock it off "stop it" (1880), which was perhaps reinforced by the auctioneer's use of the term for "dispose of quickly." To knock (someone) off in the underworld slang sense of "kill, murder" is from 1919. See knock (v.) + off (adv.).