[hwak, wak]
verb (used with object)
to strike with a smart, resounding blow or blows.
Slang. to divide into or take in shares (often followed by up ): Whack the loot between us two.
verb (used without object)
to strike a smart, resounding blow or blows.
a smart, resounding blow: a whack with his hand.
Informal. a trial or attempt: to take a whack at a job.
Slang. a portion or share.
Verb phrases
whack off,
to cut off or separate with a blow: The cook whacked off the fish's head.
Slang: Vulgar. to masturbate.
whack out, Slang. to produce quickly or, sometimes, carelessly: She whacks out a short story every week or so.
out of whack, Informal. out of order or alignment; not in proper condition.

1710–20; orig. dial., Scots form of thwack; cf. whang2, whittle

whacker, noun

5. try, go, turn.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
whack (wæk)
1.  to strike with a sharp resounding blow
2.  informal (Brit) (usually passive) to exhaust completely
3.  informal (tr; usu foll by in or on) to put something on to or into something else with force or abandon: whack on some sunscreen
4.  slang (US) (tr) to murder: if you were out of line you got whacked
5.  a sharp resounding blow or the noise made by such a blow
6.  informal a share or portion
7.  informal a try or attempt (esp in the phrase have a whack at)
8.  informal out of whack out of order; unbalanced: the whole system is out of whack
9.  an exclamation imitating the noise of a sharp resounding blow
[C18: perhaps a variant of thwack, ultimately of imitative origin]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

"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.
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

whack definition

According to arch-hacker James Gosling, to "...modify a program with no idea whatsoever how it works." (See whacker.) It is actually possible to do this in nontrivial circumstances if the change is small and well-defined and you are very good at glarking things from context. As a trivial example, it is relatively easy to change all "stderr" writes to "stdout" writes in a piece of C filter code which remains otherwise mysterious.
[Jargon File]

The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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American Heritage
Idioms & Phrases


In addition to the idioms beginning with whack, also see have a crack (whack) at; out of kilter (whack).

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
And one sip of it revealed that it was out of whack again and too acidic.
In other words, their optimal circadian sleep periods are out of whack with
  their actual sleep schedules.
In the past decade or so, the travel equation's been out of whack.
Every few yards he rose high in the water to whack a limb out of our path, then
  sank away beneath a boil of bubbles.
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