BARNACLE,
a good Job, or a Snack easily got; so called from the Gratuity given to Jockeys, for buying and selling Horses.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Barnacle in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).