RAT,
a drunken Man or Woman taken up by the Watch, and carried by the Constable to the Compter. To smell a Rat, To suspect a Trick.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
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Rat in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).