knackers

   the testicles

   A knack was a toy or small object, made by a knacker, whence a saddler, who bought old or dead beasts for their hides, whence his modern counterpart who disposes of dead cattle. The use may come from the meaning small objects but Dr Wright is persuasive when he gives:

    Two flat pieces of wood or bone... Of unequal length. (EDD)

   Partridge suggests 'Prob. ex dial knacker, a castanet or other "striker"' (DSUE) and the imagery from the small Spanish chestnut is attractive, although unconvincing.

   Those who, in their exhaustion, claim to be knackered are likely to consider themselves candidates for the knacker's yard rather than winded by a blow to the testicles.