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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
 
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