Is it ensure, insure, or assure?
early 15c., "free from mental or moral pollution, pure," from a figurative use of Latin immaculatus "unstained," from assimilated form of in- "not, opposite of" (see in- (1)) + maculatus "spotted, defiled," past participle of maculare "to spot," from macula "spot, blemish." The literal sense of "spotlessly clean or neat" in English is first attested 1735. Immaculate Conception is late 15c., from Middle French conception immaculée (late 15c.); declared to be an article of faith in 1854.