Green’s Dictionary of Slang

bumper n.1

1. (UK Und.) a pickpocket’s assistant who bumps into the victim.

[Aus]Baker Popular Dict. Aus. Sl. (2nd edn).

2. a thug, a pimp.

[UK]J. Worby Spiv’s Progress 120: Rosy’s a bad un - she and her black bumper .

3. (Irish) an amateur flat race [? contestants may fall off with a bump or bump each other’s horses].

[Ire]R. Smith Vincent O’Brien, The Master of Ballydoyle n.p.: The race that he chose for his first gamble was the Sportman’s Plate, an amateur flat race – or a ‘bumper’, as styled in Ireland [BS].
[Ire]Leinster Leader (Naas, County Kildare) 24 Oct. n.p.: Bumpers are very popular with the betting public [BS].