Knackers

British slang for testicles since the 19th century or perhaps a little earlier, deriving from the Middle English word knack, now obsolete, meaning to make a sharp noise. In the 16th century for example, knackers would have referred to castanets because of the noise they make. The transition to testicles is not beyond imagination, but without the noise, one suspects. A knacker as in knacker’s yard was a tradesman who bought old, worn out horses for slaughter for their hides and meat and dates from the early 19th century.