Green’s Dictionary of Slang

a sovereign.

1829		‘Pickpocket’s Chaunt’ (trans. of ‘En roulant de vergne en vergne’ in Vidocq 1829) IV 261: To his clies my hooks I throw in, [...] And collar his dragons clear away.
1830		Egan Finish to the Adventures of Tom and Jerry (1889) 103: He was off like a shot to pocket the ‘dragons!’.
1859		Matsell Vocabulum.
1896		‘Thieves’ Sl.’ Gent.’s Mag. CCLXXXI Oct. 349: In the earlier Victorian era ‘dragon’ was a not uncommon term for the same coin.