SYFFUF is short for "Shut your fucking face, uncle fucker", a commonly used phrase in the USA. It was introduced in 1890 by Pope Gregorius XIV but gained popularity when Matt Stone and Trey Parker used it for a song in their popular television series "South Park".
Aunt: "Bob, you suck!"
Bob: "SYFFUF!"
by Burn666 January 16, 2008
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