Is it ensure, insure, or assure?
Old English weddian "to pledge, covenant to do something, marry," from Proto-Germanic *wadjojanan (cf. Old Norse veðja "to bet, wager," Old Frisian weddia "to promise," Gothic ga-wadjon "to betroth"), from PIE root *wadh- "to pledge, to redeem a pledge" (cf. Latin vas, genitive vadis "bail, security," Lithuanian vaduoti "to redeem a pledge"). Sense remained "pledge" in other Germanic languages (cf. German Wette "bet, wager"); development to "marry" is unique to English. "Originally 'make a woman one's wife by giving a pledge or earnest money', then used of either party" [Buck]. Related: Wedded; wedding.