Is it ensure, insure, or assure?
Old English þawian, from Proto-Germanic *thawojanan (cf. Old Norse þeyja, Middle Low German doien, Dutch dooien, Old High German douwen, German tauen "to thaw"), from PIE root *ta- "to melt, dissolve" (cf. Sanskrit toyam "water," Ossetic thayun "to thaw," Welsh tawadd "molten," Doric Greek takein "to melt, waste, be consumed," Old Irish tam "pestilence," Latin tabes "a melting, wasting away, putrefaction," Old Church Slavonic tajati "to melt"). Related: Thawed; thawing.
c.1400, from thaw (v.). Figurative sense of "relaxation of political harshness or hostility" is recorded from 1950, an image from the "Cold War."