GOB,
the Mouth; also a Bit or Morsel; hence Gobbets, now in use for Bits; Gift of the Gob, a wide, open Mouth; also a good Songster, or Singing Master.
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Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
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Gob in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).