The word is a combination between the two words "fucking" and "nigger". Its origin wasn't meant offensive to anyone, only for
private use. The guys who originally used it only used it for
lulz. It could also be associated with expressing hate towards the
negro race. This is not its favored use, though
"Fnnnnnnnnnnnnnnnnn..." (Where they don't
directly say the word "fnigger", but
sort of refer to it)
