Green’s Dictionary of Slang

an ambush, a hi-jack; an assassination; also attrib.

1939		L. Short Raiders of the Rimrock 85: It’s root hog or die now, Lugan! Hell, what’s stoppin’ Sands from havin’ the whole lot of us beefed? If this country is runnin’ under bushwack law now we might’s well plan to fight it. [Ibid.] 86: A stranger had figered he could take over a whole county by bushwack.
1940		L. Short Brand of Empire 199: I’ll include you in on our Mutual Bushwhack Society too. [Ibid.] 211: Senator Waranrode is aimin’ to hang this bushwhack on me.
1980		S. Fuller Big Red One 14: ‘Their Lights on the beach are a good sign.’ [...] ‘Or a welcoming bushwhack.’.
2000		J.J. Connolly Layer Cake 250: That’s why he put the thing together in Amsterdam, the bushwhack.
2003		H. Hill Good Fella’s Guide to N.Y. 10: You don’t want to find out what ‘Bushwick bushwack’ is.