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c.1300, chastisen, from Old French chastiier "to warn, advise, instruct; chastise, admonish; punish; dominate, tame" (12c., Modern French châtier), from Latin castigare "to set or keep right, to reprove, chasten, to punish," literally "to make pure" (see castigate). Or perhaps from Middle English chastien (see chasten) + -ise, though this would be early for such a native formation. The form of the modern word "is not easily accounted for" [OED]. Related: Chastised; chastising.
He alone may chastise who loves. [Rabindranath Tagore, "The Crescent Moon," 1913]