Not to be confused with whak, whak? (with question mark) is short for "what the fuck?" It first appeared in this context on 30 Rock on 3/25/2010, and was uttered by the inimitable Tina Fey, expressing disbelief. It is also ideally suited for Twitter, collapsing 14 characters into 5. Finally, it requires no additional modifiers, standing on its own.
Whak? As in, what the fuck?