The word meaning very bad or of dubious quality is wack, with no h. Your spell check might disapprove of wack, but the word has been in English at least a quarter of a century, and it has another sense—an eccentric or crazy person—that is even older, so spell check is wrong. In the newer sense, the word arose out of U.S. hip-hop culture, probably around 1980. More specific origins have not been definitively established. It could descend from the older adjective wacky, which means peculiar or odd.
Whack‘s main definitions are (1) to strike with a sharp blow, (2) a sharp blow, (3) the sound of a sharp blow, and (4) to kill someone deliberately. It’s also in the idioms take a whack at, meaning to try; out of whack, meaning improperly balanced or not functioning correctly; and whack off, whose meaning you can find elsewhere. Whac-a-mole is properly so spelled as that’s how the name of the game is spelled, though it’s often spelled wack-a-mole or whack-a-mole.
Given the older definition of wack, this spelling might seem to make sense in wack-job, which refers to an eccentric or crazy person, but that term is usually spelled whack-job. Perhaps we are to infer that a whack-job is someone who has been whacked in the head, figuratively or otherwise.
The Weeknd took his fellow Canadian friend Drake’s side in his rap spat against Common by basically calling Common’s recent comeback diss track wack. [Singersroom News (link now dead)]
[H]e discussed the state of hip-hop, Tupac, and how he thinks hip-hop is wack because of “old white executives.”
Paul, however, has given critics ample grounds to think he’s a wack job. [Chicago Tribune]
If prices are out of whack, they are more aggressively matching the prices of their online rivals. [Wall Street Journal]
Charlie Sheen says he is no longer the total whack job we saw at this time last year. [SheKnows]
Our dear friend Beekers most eloquently addressed this sentiment already, but allow me to take a whack at it. [Field Gulls]
The guy was standing in front of him and he whacked him, severely, on the leg with his goal stick. [quoted in Kingston Whig-Standard]
Nor, by the way, do I think that Clinton, Queen Elizabeth, or Sarkozy had him whacked. [NPR]