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bushwhacker (n.)[bushwhacker 词源字典]: also bush-whacker, 1809, American English, literally "one who beats the bushes" (to make his way through), perhaps modeled on Dutch bosch-wachter "forest keeper;" see bush (n.) + whack (v.). In American Civil War, "irregular who took to the woods" (1862), variously regarded as patriot guerillas or as freebooters. Hence bushwhack (v.), 1837; bushwhacking (1826).[bushwhacker etymology, bushwhacker origin, 英语词源]
whack (v.): "to strike sharply," 1719, probably of imitative origin. The noun is from 1737. The word in out of whack (1885) is perhaps the slang meaning "share, just portion" (1785), which may be from the notion of the blow that divides, or the rap of the auctioneer's hammer. To have (or take) a whack at something "make an attempt" is from 1891. Related: Whacked; whacking. Whacked out is from 1969.