usually in past tense, knackered, "to kill, castrate" (1855), but most often used in weakened sense of "to tire out" (1883); apparently from knacker (n.) "worn-out or useless horse," 1812, of unknown origin; possibly from a dialectal survival of a Scandinavian word represented by Old Norse hnakkur "saddle," hnakki "back of the neck," and thus possibly related to neck (n.).