Green’s Dictionary of Slang

rubber n.1

[? SE rub (up against); ? link to the sporting use, coined in the 16C, meaning a match, adopted in the 17C as a quarrel or fight]

1. [mid-16C] (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.

2. [early 17C; 1900s–50s] any form of deception or trick.