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1.
A person who greatly enjoys reading about or watching the fictional character Hannibal Lecter, created by Thomas Harris. This enjoyment often borders on obsession. Fannibals can be found lurking in the dark recesses of the internet and are intensely devoted to the books by Harris, the movies, or the new television series.
Haley is such a Fannibal that she has an entire tumblr devoted to the Brian Fuller "Hannibal" series.
by pleasegodmakemegood April 20, 2013
 
2.
an avid fan of Hannibal. Obsessed in whether books (by Thomas Harris), movies and/or tv series. Fannibals are a really polite fandom and they prefer to eat bad people (hence the motto 'Eat The Rude'). Some say they're the baby brother of the Sherlock fandom.
"if the Doctor Who fandom call themselves Whovians, what are the Hannibal fans called?"

"Fannibals!"
by DeerGod June 27, 2013
 
3.
One who is so devoted to a person, place, or thing that they spend almost every waking minute thinking about that thing, they have posters and pictures of it all over their room and/or house, and always have the latest news on it.
"John is such a fannibal about the Backstreet Boys."

"Jill is fannibalistic about alligator slippers!"
by Teej "Godfajah" Osburnicus November 08, 2006
 
4.
Someone who eats a lot of pussy (British) or ass (American) It's a combination of cannibal and fanny. Fanny is slang for pussy in the UK and bum in the US.
UK: Mike eats more pussy than cervical cancer, he's a real fannibal!
by Dries van Staal December 19, 2006