The testicles of a man or beast; also a vulgar nick name for a parson. His brains are in his ballocks, a cant saying to designate a fool.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Balderdash * Balum RancumNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes