What does the 6666 devil sign mean?

No, it is a mistranslation of hundreds of years worth of Chinese whispers. There is a 'number of the beast', written as 616, not 666.

There are many other 'signs' of the devil, most of which today are null and deriven from local folklore and fairytales. Many original pictograms and visual symbolism used originally are long gone, or still present merely as an influence on modern signs.