Is it ensure, insure, or assure?
c.1200, "clear (a space); set free, save," from a Scandinavian source akin to Old Norse ryðja (past tense ruddi, past participle ruddr) "to clear (land) of obstructions," from Proto-Germanic *reudijanan (cf. Old High German riuten, German reuten "to clear land," Old Frisian rothia "to clear," Old English -royd "clearing," common in northern place names), from PIE root *reudh- "to clear land." The general sense of "to make (something) free (of something else)" emerged by 1560s. Senses merged somewhat with Northern English, Scottish, and U.S. dialectal redd. To get rid of (something or someone) is from 1660s. Related: Ridden; ridding.