“Wack” has been an accepted term for someone who is a bit odd for a long time (originating roughly in the 1930s), most likely coming from the adjective “wacky.” More recently (roughly the 1980s), it became a term for something of dubious quality or something inferior or bad. It’s slang, sure, but when it has a unique spelling, you need to make sure you’re not misusing it when you mean something else.
“Whack” (with the “h”) has a few definitions from striking forcefully to the strike itself to murdering and beyond, but that “h” is necessary in all of these forms.
And here’s a twist: remember that old arcade game where you whack moles with a big mallet? That was called Whac-a-mole. I know. It’s wack.
Thus ends today’s random lesson in word confusion.