Green’s Dictionary of Slang

(UK und.) one who encourages gulible victims to play a game of thimble-rig n. (1)

1827

J. Wight More Mornings in Bow St. 42: Every operator in this game is attended by certain of his friends called eggers and bonnnetters — the eggers to egg on the green ones to bet, by betting themsleves; and the bonnetters, to bonnet any green one who may happen to win, that is to say, to knock his hat over his eyes while the operator and the others bolt with the stakes .