Stands for "Don't fucking lie, faggot". Used when someone on the internet tries to make themselves look awesome, like they have a life, or sometimes even pitiful, on the internet.
Kirsty: "Get the sand out of your vagina, you fat whore."
Sarah: "I'm only 120 lbs, and I have the body of a model!"
Kirsty: "DFLF. You leave footprints in concrete."

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Gareth: "Ashuri doesn't love me! I'm gonna kill myself!"
Chad: "DFLF. You've 'killed yourself' 20 times tonight already."
by Johnnicles August 8, 2009
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