An imaginary illness. He has got the humdurgeon, the thickest part of his thigh is nearest his a-se; i.e. nothing ails him except low spirits.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hum Drum * HumbugsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes