Most common in fixed combinations: 独门绝技 (unique signature skill), 独门秘方 (exclusive secret formula), 独门生意 (a business no one else has). Strong wuxia-novel and traditional-craft flavor - kung fu schools and old family restaurants are typical settings.
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独 (alone / sole - as in 独立 independent) + 门 (gate / school / lineage - same 门 as in 武术门派 'martial arts school'). Together: 'sole school / single house' = a technique kept inside one lineage.
