FF means many things, some already said, I might (most likely will) repeat.
#1. Fist fucking- the act of stuffing a fist up a pussy *owch*
#2. Foot fucking- a type of masturbation that women do to their male partners dick.
#3. Final fantasy- a game played by geeks, such as my boyfriend, that I should probably learn to play.
#4. Fan Fiction- a type of writing that uses already made characters from books and movies and such that have sex, also called slash... gay LOTR ones at www.libraryofmoria.com.
#5. Father fucker- a person who likes to fuck their dad/friend's dad.
#6. Fucking freak- ... me...

heres all the definitions for FF:

freindly fire
Fire fight
Flying fuck
Flaming Fist
Frostfire
fortissimo
focus fire
Front Wheel Drive
Forum faggot
Freedom Fries
finds funny
False Friend
Fake fuck
Foot Fucker
Fucking Fool
Fan Fiction
Final Fantasy
Front flip
fun fuck
Fucking freak
Father fucker
Fist fucking
Fat Fuck
Freshly fucked
Fatal Fury
Final Fight
Front Engine

ff is texting short term for "fucking funny." Much more hip than an lol!

Freshly fucked.

Someone who just wants to "fuck your face" aka "face fucker(s)" or "ff" on dating sites

"ff" is a term used to define what kind of porn you're going to find in the link it gives you. "ff" refers to female on female action. get ready for the lesbians!