To SILENT a Man,
to knock him down, so as to stun him: To lay him down for dead. See the Cull is Silent, is also us'd by desperate Villains, for cutting the Throat, or shooting the unhappy Person who falls in their way.
The Vulgar Tongue originally collected by Grose, 1811.
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).