Among bikers 666 represents the 6th letter of the alphabet. Stands for FFF - Filthy Few
Forever. Originated by (?) the Hells Angels and was used for it's shock value as society correlated it with being the "Number of the Beast".
Some 1% bikers wear the 666 patch to indicate they are "pure evil" and have done many things society would view as "evil". They are generally "the baddest of the bad".
Not usually a spoken term but used on patches on the biker's cut
This patch is found either in the shape of a rhombus (diamond) or as a "rocker".
Did you see the 666 on his cut
? Man, he is 666 (pure evil).