HUSH Money,
Money given to hush up, or conceal a Robbery or Theft, or to take off an Evidence from appearing against a Criminal, &c.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
Hush Money in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).