Green’s Dictionary of Slang

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

1943

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

2. a thug, a pimp.

1939		J. Worby Spiv’s Progress 120: Rosy’s a bad un - she and her black bumper .
1946		D. Burley N.Y. Amsterdam News 14 Dec. 21: Big Freddie, the Bumper [...] the No Good man.

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

1990		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].
1996		Leinster Leader (Naas, County Kildare) 24 Oct. n.p.: Bumpers are very popular with the betting public [BS].