Crackfuck is the joining of Crackhead with fucktard, producing "Crackfuck." It is popular because of the alliteration of the hard k sound in Cra, ack, uck.
A type of fiction where it is believed that the author must be on crack to have written such a story because it's so bizarre or off the wall Often, a facial expression after reading one of these crackfics is 'O.O' or 'O.o'
"I just read a crackfic where Chuck Norris dreams he's Luke Skywalker than becomes a stripper to make more money for his kid"