1Particularly common in 17th century Of a person, disposition, etc.: not susceptible to or influenced by passion or strong emotion; calm, self-possessed; (less neutrally) unfeeling, bloodless.
2Particularly common in 17th century Of action, speech, etc.: not expressive of or characterized by strong emotion; (of an emotion or quality) rational, moderate.
Late 16th century; earliest use found in Philip Sidney (1554–1586), author and courtier. From un- + passionate.