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the person concerned; the person in question; the said person
当人がいないところで噂話をするな。
Do not gossip when the person in question is not present.
当人の意志を尊重することが大切だ。
Respecting the person's own wishes matters.
結婚は当人同士が決めるべき問題だ。
Marriage is a matter to be decided by the parties involved.