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.