This is from The Fanfiction Glossary (http://www.subreality.com/glossary/terms.htm#C):

"crackfic - A fanfic whose concept is so out there, one wonders what the author was snorting when they wrote. Sometimes deliberate, sometimes just 'WTF?'"

Here's (http://peskywhistpaw.livejournal.com/126195.html#cutid1) an example of a crackfic.