Green’s Dictionary of Slang

1. (UK Und.) a member of a team of confidence tricksters who works as a back-up to those running the fraud; if the victim realizes they are being tricked, the rubber swiftly causes a disturbance, usu. by picking a fight with the earnest bystander, thus allowing their confederates to grab the stakes and run.

1552

G. Walker Detection of Vyle and Detestable Use of Dice Play in Judges (1926) 48: If, when the money is lost, the cousin begins to smoke, and swear that the drunken knave shall not get his money so, then standeth the rubber at the door, and draweth his sword.

2. any form of deception or trick.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 55: rubber, n. A story of action intended to deceive.
1901		W. Irwin Love Sonnets of a Hoodlum VI n.p.: Rubber, thou scab! Don’t throw on so much spaniel!
1937–84		Partridge DSUE (1984) 993/1: early 17C.