"to kill, castrate" (1855), apparently from knacker (n.) "one who slaughters old or sick horses" (1812). This is probably the same word as the earlier knacker/nacker "harness-maker" (1570s), which survived in 18c. in dialects. The sense extension is perhaps because knackers supplied farmers general help with horse matters, including disposing of dead ones. The word is of uncertain origin, possibly from a dialectal survival of a Scandinavian word represented by Old Norse hnakkur "saddle," related to hnakki "back of the neck," and thus possibly related to neck (n.). Most often used in weakened sense of "to tire out" (1883) and usually encountered in its past tense, knackered.