"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.
[probably echoic; in second verb sense, the use of whacks, ''any form of force,'' is attested among Chicago gunmen in 1932]
[University of Maryland: from hacker] 1. A person, similar to a hacker, who enjoys exploring the details of programmable systems and how to stretch their capabilities. Whereas a hacker tends to produce great hacks, a whacker only ends up whacking the system or program in question. Whackers are often quite egotistical and eager to claim wizard status, regardless of the views of their peers. 2. A person who is good at programming quickly, though rather poorly and ineptly.