Green’s Dictionary of Slang

(UK Und.) to turn someone upside down and shake them until the money falls out of their pockets.

c.1698		B.E. Dict. Canting Crew n.p.: Revers’d, a Man set (by Bullies) on his Head, and his Money turn’d out of his Breeches.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Reversed, A man set by bullies on his head, that his money may fall out of his breeches, which they afterwards by accident pick up.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum.
1895		C. Crowe Aus. Sl. Dict. 66: Reversed, a person made to stand on his head by rowdies, so that his money may fall from his pockets; it is picked up as money found.