Is it ensure, insure, or assure?
c.1300, noun use of adverbial phrase par amour (c.1300) "passionately, with strong love or desire," from Anglo-French and Old French par amour, from accusative of amor "love," from amare "to love" (see Amy). Originally a term for Christ (by women) or the Virgin Mary (by men), it came to mean "darling, sweetheart" (mid-14c.) and "mistress, concubine, clandestine lover" (late 14c.).