Green’s Dictionary of Slang

1. (orig. US) very drunk.

1960

(con. 1940s) D. MacCuish Do Not Go Gentle (1962) 82: She’s plastered, Normy. Bushwacked! Taken in the rear, by booze.

2. ambushed, hi-jacked.

1957		H. Ellison ‘Look Me in the Eye, Boy!’ in Deadly Streets (1983) 157: We was bushwhacked.
2000		J.J. Connolly Layer Cake 115: Some very capable geezers who may be thinking that we’re here to mug them off a lorry load of bushwhacked pills.