Green’s Dictionary of Slang

1. a good job or quick profit easily obtained.

c.1698		B.E. Dict. Canting Crew.
1725		New Canting Dict.
1750–1882		B.M. Carew Life and Adventures.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1895		Crowe Aus. Sl. Dict. 6: Barnacles [...] a good booty.

2. a tip given to a groom at a horse sale.

c.1698		B.E. Dict. Canting Crew.
1725		New Canting Dict.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

3. (Aus.) one dollar.

1991

J.T. Pickle Aus.-Amer. Dict. 22: BARNACLE: Term for a dollar when gambling.