Often used in internet slang or the language more commonley know as "Leet".
The sixes represent "g's"
in simple English the word is defined as meaning nigger or nuggah.
Often used as a way of describing friendship or used in a deragatory way depending on context.
"AOL System Manager: What Up Nu66ah?"
"AOL System Manager: What the hell dude? You're a nu66ah"