Green’s Dictionary of Slang

1. one who covers for an accomplice.

1821

D. Haggart Autobiog. 66: The Doctor [...] played the part of the gammoner so well, that I made my escape without being observed.

2. one who ‘spins a yarn’ or tells deceitful tales; thus a prime gammoner, an expert at such tale-spinning.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 242: gammon: [...] a man who is, ready at invention, and has always a flow of plausible language on these occasions, is said to be a prime gammoner.
1821		W.T. Moncrieff Tom and Jerry I i: Acquire just enough experience to make you [...] fly to the gammoners, and awake to everything that’s going on.
1832		Egan Bk of Sports 54: Counsellor Phillips, as good a gammoner as ever sailed down the Liffey.
1847		Bell’s Life in Sydney 13 Feb. 3/4: We have the honour to remain, A. Gammoner, B. Blarney, C. Yarnwell, U Knowall, &c., &c.
1848		Flash Dict. in Sinks of London Laid Open.
1953		Randolph & Wilson Down in the Holler 247: gammoner: n. A talkative, unreliable person.