"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.
Very big; whopping: After that she got a whacking big raise (1806+)
The human brain: Slip a microchip into snug contact with your gray matter (a.k.a. wetware)/ wetware: the human brain and its DNA code (1980s+ Computer)