The word "whack" does not have an adjectival version ending in "y" and meaning "crazy" or "silly," as is seen in this blog comment. The word wanted is "wacky." Note the spelling -- "w" is used, not "wh." (Careful speakers would pronounce the two words differently -- "wh" is unvoiced while "w" is voiced -- but today it's becoming increasingly rare to find people who pronounce "wh" and "w" differently.) "Whack" is a verb meaning "to strike" or "hit." Now don't misuse these two words anymore or I'll whack you in the head until you're wacky.