One of the mysteries of the English language finally explained.
A psychiatric disorder characterized by the existence in one person of two or more distinct personalities which are capable of taking total control of the person's behaviour at different times, and between which the person typically switches suddenly (also "multiple personality disorder"). Also: a person suffering from this disorder; (in plural) the various personalities exhibited by such a person.
Late 19th century. From multiple + personality.
multiple personality/ˌmʌltɪpl ˌpəːsəˈnalɪti/
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