Mr. Hickenbothom; a ludicrous name for an unknown person, similar to that of Mr. Thingambob. Hickenbothom, i.e. a corruption of the German word ickenbaum, i.e. oak tree.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hick * HickeyNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes