Mr. Thingumbob; a vulgar address or nomination to any person whose name is unknown, the same as Mr. What-d’ye-cal’em. Thingumbobs; testicles.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Thingstable * ThirdingNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes