The number 6 is probably a reference to classic hexagram, which is something used to put a curse on someone, sort of like the targeting reticle of a gunner.
Used in common speech as a reference to eradicating a group of (usually valiant) people.
It has recently surfaced in popular culture in the George Lucas film Star Wars Episode III: Revenge of the Sith, when Emperor Palpatine orders his Stormtroopers to "execute order 66", which is the instruction to kill all Jedi.
Clone General: It will be done, my Lord.
2.What is yelled when nerds at a burger joint notice that the order ticket for a customer has the number 66 on it,in a nerdy reference to star wars.