666 is live-stream and gaming slang. It plays on 六 (liù) ≈ 溜 (liū, 'slick / smooth') — so '666' reads as '溜溜溜', meaning someone is playing slick. Typed in bullet-chat (弹幕) when a streamer does something impressive.
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Casual / online only. Never use in formal writing. Note the pinyin is 'liùliùliù' — Chinese speakers read the digits out, not as 'six six six'.
