FERRET,
a Parn-broker, or Tradesman that sells Goods upn Trust at excessive Rates, and then hunts them, and often throws them into Goal, where they perish for his Debt.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Ferret in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).