Green’s Dictionary of Slang

jump on v.

also jump upon

1. to attack, verbally or physically, someone who is seen to have exposed themselves to such an assault by their behaviour or their weakness.

[UK]M.E. Braddon Dead Sea Fruit v: When a wretched scribbler was, in vulgar phraseology, to be jumped upon, honest Daniel put on his hobnail boots and went at the savage operation with a will [F&H].
[US]Lantern (N.O.) 1 Oct. 2: The idea of two big chaps jumping on one man.
[UK]G.R. Sims ‘A Plea for Mercy’ Dagonet Ditties 77: Do not flog the brutal rough / Who jumps upon his wife.
[UK]R. Whiteing No. 5 John Street 91: ‘Oh, go on jumpin’ on me,’ returns Low Covey.
[US]J. Flynt World of Graft 84: There is probably a fair amount of crookedness in the Police Department, who is to be jumped on before such conditions can be changed?
[US](con. 1920s) S. Lewis Elmer Gantry 391: I’ll jump on the police for not having pinched these places.
[US]C. McKay Home to Harlem 47: They done jumped on me soon as I turned mah black moon on that li’l saloon.
[US]W.R. Burnett Dark Hazard (1934) 154: She jumped on him for gambling and being irresponsible.
[US]W. Guthrie Bound for Glory (1969) 178: They quit jumping on me for two reasons: I’d beat the hound out of them, and the others wanted to ride on that motorcycle.
[US](con. 1950s) D. Goines Whoreson 98: I didn’t want to jump on her.
[US]R. De Christoforo Grease 97: Why you guys jumping on my number today? Just cause I laid it out on Sandy, don’t mean you got to get on me.
[US]T.R. Houser Central Sl. 32: jumped-on [...] An aggressive roughing up.

2. see step on v.