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1.
The exact origin of this one is unknown (at least to me). However, FFFFF or "F-Lock" is an expression of discontent, possibly a way to avoid saying the word "fuck" by simply drawing out the F.
"Knock knock"
"Who's there?"
"Joe."
"Joe Who?"
"Joe Momma!"
"FFFFF."
by GreyDS February 14, 2009
 
2.
The sound someone makes, right before they're about to laugh very loudly.
Bob: Hey guess what?

Dan: What?

Bob: Lady Gaga just got hit it the face with a pie!

Dan: Fffff- Hahahaha~!
by Bob/Dan March 31, 2010