Is it ensure, insure, or assure?
mid-15c., from Middle French furniss-, present participle stem of furnir "furnish, accomplish," from Old French fornir (12c.), from Vulgar Latin *fornire, alteration of *fromire, from West Germanic *frumjan "forward movement, advancement" (cf. Old High German frumjan "to do, execute, provide"), from Proto-Germanic *fram- "forwards" (see from). Meaning "to provide" (something) is from 1520s. Related: Furnished; furnishing.