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".
A small piece of information. Derived from the word ken, used often in the scottish language and is synonymous with knowledge.
Person 1: "Hey I don't get this shit. How do you solve this problem?"
Person 2: "I got that one. Give me some kenlets on this assignment and I'll help you w/ that one."