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 cuts.
This patch is found either in the shape of a rhombus (diamond) or as a "rocker".
"...And that number is 666."
Widely implied that it was 666 because it was an imperfect number to represent the impurities of man and their evildoings and sinful ways.
Read the book of revelations, at the end of the New Testament; it tells all about that, the seven seals, the apocalypse, Armageddon, Megiddo, etc...