Is it ensure, insure, or assure?
late Old English mægester "one having control or authority," from Latin magister (n.) "chief, head, director, teacher" (source of Old French maistre, French maître, Spanish and Italian maestro, Portuguese mestre, Dutch meester, German Meister), contrastive adjective ("he who is greater") from magis (adv.) "more," from PIE *mag-yos-, comparative of root *meg- "great" (see mickle). Form influenced in Middle English by Old French cognate maistre. Meaning "original of a recording" is from 1904. In academic senses (from Medieval Latin magister) it is attested from late 14c., originally a degree conveying authority to teach in the universities. As an adjective from late 12c.
early 13c., "to get the better of," from master (n.) and also from Old French maistrier, from Medieval Latin magistrare. Meaning "to reduce to subjugation" is early 15c.; that of "to acquire complete knowledge" is from 1740s. Related: Mastered; mastering.