Green’s Dictionary of Slang

a Spanish dollar.

c.1687		‘Mac O Bonniclabbero of Drogheda’ Bog Witticisms LXXV 102: The priest knowing him to be a Rich Fellow, expected a Sum of Money from him (which the Young Man well knew also) and did bring a Bagg of Forty Cobbs; which are as much as Five Shillings every one of them.
1690		‘Teague’ Teagueland Jests II 151: You have lodged in my House above six Weeks, and I have not received one Cob of you.
c.1698		B.E. Dict. Canting Crew n.p.: Cob a Dollar (in Ireland).
1725		New Canting Dict. n.p.: cob an Irish Dollar.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Cob, a Spanish dollar.
1805		Plymouth Newspaper 24 Feb. quoted in Earl of Dundonald Autobiog. of a Seaman I 174: [...] Spanish ‘pewter’ and ‘cobs,’ nicknames given by seamen to ingots and dollars [F&H].
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
c.1850		Duncombe New and Improved Flash Dict.