In this term, "fuck" is a measure of the word "bad". In simpler terms, if there were a scale for bad words, "damn" would be on one end and "fuck" would be on the other. Therefore, fuck would be as bad as you can get. Also, "bad" is used as slang for "good".
Why don't Churches Chicken start sellin' watermelon? That would be bad-as-fuck, yo!