The term was popularised by historian Wu Si (吴思) in his 2001 book 《潜规则：中国历史中的真实游戏》. In post-2000s usage it carries strongly negative connotations - most often the casting-couch sense in entertainment, or kickbacks and favouritism in business.
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Common in news and gossip; in everyday conversation 走后门 ('go through the back door') covers similar ground for the favours/bribery sense.
