Green’s Dictionary of Slang

a golden sovereign.

1895		Crowe Aus. Sl. Dict. 18: Dragoons, gold coins [...] Collar His Dragoons, seize his sovereigns.
1901		Argus (Melbourne) 20 Sept. 6/4: A sovereign is [...] a goldfinch, a dragoon, a king s picture, a poona, or a thick ’un.