Green’s Dictionary of Slang

gammon v.

[? game v. (3) or SE (back)gammon or fig. tying up of a gammon or ham; NB ‘Sl. Terms & the Gypsy Tongue’ in Baily’s Mag. Nov. 1871 suggests origin in Hindi ghumana, to beguile]

1. [late 17C–early 18C] to cheat (at a game).

2. [mid-18C] (UK Und.) to help.

3. [late 18C] to talk criminal slang.

4. [late 18C+] to deceive, to fool, to talk humbug, to pretend.

5. [19C] to persuade.

6. [mid-19C] to tease amicably.

7. to flatter.

In phrases

gammon a maim (v.)

[early 19C] (UK Und.) of a beggar, to pose as ill.

gammon the draper (v.)

[early 19C] ‘When a man is without a shirt, and is buttoned up close to his neck, with merely a handkerchief round it, to make an appearance of cleanliness, it is termed, “gammoning the draper”’ (Egan, Life in London, 1821).

gammon the twelve (in prime twig) (v.) [SE twelve, generic for the jury + in fine twig under twig n.1 ]

[late 18C–early 19C] (UK Und.) to gain an acquittal in court; the implication is that the defendant has managed to fool the jurymen.