It seems that the origin of the word "whacked" in common parlance arose from the phrase "whacked in the head." This is how it was said a few years ago. "She acts like she was whacked in the head," or "did you get whacked in the head?" It meant, that he/she/you acted as though suffering from a concussion, the result of being hit hard enough in the head to cause brain trauma. From there, it degenerated to "he's acting whacked," and from there to "she's whacked," or the even more derivative "that's just whacked," to describe a thing or situation so bad it seems to be something one might hallucinate if suffering from said concussion.

Now we get to the actual abuse of the language. The annoyingly common usage now is "whack," as in "she's whack." Whack in this case is intended to be used as an adjective, but the language used is for the verb. It comes from the same laziness that creates such classics as "aks," as in "she aks him a question," or even "ast" as in "I ast you a queshun."

Whack is a verb, as in "it is good to whack things" or "whack him!" It is a mixture of laziness, ignorance, and bad grammar to say "he's whack." Come on, people!