Brit.
¦ noun
1. a person who disposes of dead or unwanted animals.
2. (knackers) vulgar slang testicles.
¦ verb [often as adjective knackered] informal exhaust; wear out.
?damage (something) severely.
Origin
C16 (orig. denoting a harness-maker): possibly from obs. knack 'trinket'; sense 2 may be from dialect knacker 'castanet'.