The latest slang meaning of 666 originated from online e-sport games and then caught on widely in the entire online sphere.
6 sounds the same in Chinese as 溜, which has multiple meanings. One of its meaning is as a verb -- to connotate escaping/fleeing/skating away. However, it can also be an adverb which means awesome, amazing, superb or highly skilled.
So in its origin amongst online e-sports, gamers used it to compliment other players by typing "666" as a quick way to represent "溜溜溜".
Some examples of 溜 in its proper usage:
他英语说得很溜：He can speak English fluently
他钢琴玩得很溜：He is highly proficient in playing the piano