mistaken identity; person so changed they are hard to recognize
Refers to confusing one person for another, or to someone who has changed so much that they are barely recognizable. Often used in set phrases like 人違いをする.
Sorry, I mistook you for someone else.
He has changed so much that he is barely recognizable.
Compound of 人 (ひと, 'person') and 違え (ちがえ), the nominalized stem of 違える (ちがえる, 'to mistake, to change'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning of mistaking or changing a person.