HOBBY,
as Sir Posthumus Hobby, one that draws on his Breeches with a Shoeing-horn; a Fellow that is nice and whimsical in the Set of his Cloaths.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Hobby in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).