When something is behaving inexplicably and unpredictably.
Eric, I'm tired of working on this whackass code.
when someone is whack, just a harsher version of it. comes from root word whack which means dumb or a word to define someone who is obviously exaggerating. can mean wrong. usually meant in a playful sense.
joe: i shot the president !!!
bob: man, you such a whackass.