Is it ensure, insure, or assure?
mid-14c., "to come to an end," also "to settle, decide" (late 14c.), from Old French determiner (12c.) or directly from Latin determinare "to enclose, bound, set limits to," from de- "off" (see de-) + terminare "to mark the end or boundary," from terminus "end, limit" (see terminus). Sense of "coming to a firm decision" (to do something) is from mid-15c. Related: Determined; determining; determiner.