Etymology
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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 by 1820 (with have), 1845 (with take). Wack or whack "crazy person," 1938, is probably a back-formation from wacky, which probably comes from the blow-on-the-head verb. Related: Whacked; whacking. Whacked out is from 1969.

Wack, whack in the slang sense of "unappealing; crazy," hence "worthless, stupid" is by 1986, apparently popularized by an anti-drug slogan crack is wack.

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wacky (adj.)
"crazy, eccentric," 1935, variant of whacky (n.) "fool," late 1800s British slang, probably ultimately from whack "a blow, stroke," from the notion of being whacked on the head one too many times.
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bushwhacker (n.)
also bush-whacker, 1809, American English, "woodsman, one accustomed to life in the bush," literally "one who beats the bushes" (to make his way through), perhaps modeled on Dutch bosch-wachter "forest keeper;" see bush (n.) + whack (v.).

Among Northern troops in the American Civil War, "Confederate irregular who took to the woods and fought as guerrillas" (1862). Related: bushwhack (v.), 1837; bushwhacking (1826).
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bull-whip (n.)

also bullwhip, "long, thick type of whip 'used by drovers to intimidate refractory animals'" [Century Dictionary], 1852, American English, from bull (n.1) + whip (n.). Whips made of bull hide were known in Middle English as bull-rope, bull-sinew, bull-yerde. But in bull-whip (also in 19c. American English bull-whack) the sense is perhaps "whip that can drive a bull," or bull might be a reference to the size of it. The earliest references (1850s) are in Northern accounts of Southern slavery. As a verb from 1895.

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