Green’s Dictionary of Slang

Sir Posthumous Hobby n.

an obsessive dandy.

c.1698		B.E. Dict. Canting Crew n.p.: Sir Posthumus Hobby, one that Draws on his Breeches with a Shoeing-Horn, also a Fellow that is Nice and Whimsical in the set of his Cloaths.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Hobby, Sir Posthumous’s hobby, one nice, or whimsical in his clothes.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.