This term can mean
ambush, as in "whacked while in the bush."
Etymology uncertain to this noder; I've come across references to it in the American Old West ('
bushwhackers') as well as extensive recent use in
Vietnam. In the latter, 'in the bush' was slang for 'in the jungle' as in 'not at base;'
bushwhacking therefore referred to ambush or attacks suffered while on
patrol.
Another (more literal?) use of the term describes the action of clearing a path. This may be the more direct source of the word, given the two root words that form it.