Green’s Dictionary of Slang

1. (UK Und., also jolly) a card sharp’s accomplice, who pretends to be a member of the public.

1886		W. Newton Secrets of Tramp Life Revealed 11: Sometimes he will do it very slowly and you can see where it is, but the ‘jollier’ or ‘sweetener’ will bet him and win. This is to encourage you to stake more money, and when you do, this is the time he wins.
1897		E.W. Townsend Chimmie Fadden 67: Down where I uster live I was de boss jollier wid de chippies, but dis time I was nothing but a farmer.
1914		W. Sickert New Age 19 Mar. 631: He would carry with him the ‘jollier,’ whose duty it is to keep the ‘mug’ amused, and rouse him to acts of folly.
1938		F.D. Sharpe Sharpe of the Flying Squad 194: One man only manipulates the cards. The others are known as ‘Jollies.’ Their task is to ‘jolly’ the mug into playing. [Ibid.] 199: The ‘Jollies’ on either side of him kept barging into him [...] They kept pestering him to play.

2. a flatterer.

1904		‘Number 1500’ Life In Sing Sing 250: Jollier. A flatterer.
1905		N.Y. Eve. Post 12 Oct. 2: He was talkative, and, as the attendants say, ‘quite a jollier’ [DA].