It's a wacky world, or possibly a whacky world. The word, first found in print in the 1930s, means wildly odd or eccentric. Fittingly, there is nothing straightforward in tracking the progress of wackiness.
Consider the spelling. North America has treated it as wacky. A Nov. 8 article in The Wall Street Journal, addressing California as "the Lindsay Lohan of states," wrote: "Helping enforce your wacky laws will be Lt.-Gov.-elect Gavin Newsom, the San Francisco mayor who flouted state law by allowing same-sex marriage." On the same day, The Globe and Mail referred to the "candwich: a sandwich stuffed in a pop can" as "this wacky product."
Britain, though it now seems largely to favour wacky, has been happy to use whacky. In 1964, The Economist used the phrase "a little bit whacky." In the current issue of the music magazine Mojo, the review of a 1972 movie called Gold: Before Woodstock. Before Reality says, "This farrago was dreamt up in 1968 by Merry Pranksters/ Grateful Dead associate and improv legend Del Close and whacky actor Garry Goodrow."
Nobody knows exactly how wacky came to be, but the consensus is that it derived from whack. It might have emerged from the sense of hitting somebody, administering a whack to the head so powerful that the recipient became odd or peculiar. More likely, it derived from "out of whack," which means malfunctioning or out of order and was first recorded by the Oxford English Dictionary in 1885.
That raises the question of where "out of whack" came from. Since the 1700s, a whack was a full share of something obtained by nefarious means. A dictionary of slang in the 1800s defined "to go whacks" as to divide equally. It's possible that if the proceeds weren't divided equally, they would be out of whack, and the system wouldn't be operating as it should.
On the other hand, by 1860 whack also referred to an agreement, as in, "That's a whack." The second edition of Johnson's Original Comic Songs in 1860 boasted the refrain, "I axed her for to marry me, she said it was a whack." It's possible that if an agreement were violated, it would be out of whack.
Wacky is so powerful that any other word that comes close risks being absorbed. W.A.C. Bennett, the longest-serving premier of British Columbia, was dubbed Wacky Bennett by his critics. Until the 1993 confrontation between U.S. federal authorities and the Branch Davidian cult ended horribly with the death of about 80 cult members, late-night comedians regularly conflated the Texan site of the standoff, Waco, with wacky and its variant, wacko. (Fans of the TV series Animaniacs will recall that one of the lead characters, in homage to Groucho, Chico and Harpo of the Marx Brothers, was named Wakko.)
Whack, the antecedent of wacky, appears to derive from thwack, which derived from thack, which in Old English (when it was spelled thaccian) meant to pat or hit somebody or something with the palm of the hand. All those words were imitative, which is to say they sounded like the actions of which they spoke.
Should you choose to investigate it, there is a large body of study about the degree to which words such as whack are pronounced hwack. Until the French started messing about in the Middle Ages with the way the Old English spelled words, where was spelled hwaer and why was spelled hwy.
Roy Blount Jr. takes a page in his book Alphabet Juice to argue the case for pronouncing the h. "To me, there are two whys, one as in 'Why, bless my soul,' pronounced wy, and the other as in 'Why, baby, why?', pronounced hwy. Highly literate friends of mine don't hear the difference. I'll bet dogs do." There's your choice. Side with the highly literate or with the dogs. English can be wacky sometimes.