A musical instrument made of a mopstick, a bladder, and some packthread, thence also called a bladder and string, and hurdy gurdy; it is played on like a violin, which is sometimes ludicrously called a humstrum; sometimes, instead of a bladder, a tin canister is used.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hummums * HumpNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes