Is it ensure, insure, or assure?
c.1300, from Old French afermier (Modern French affirmer) "affirm, confirm; strengthen, consolidate," from Latin affirmare "to make steady, strengthen," figuratively "confirm, corroborate," from ad- "to" (see ad-) + firmare "strengthen, make firm," from firmus "strong" (see firm (adj.)). Spelling refashioned 16c. in French and English on Latin model. Related: Affirmed; affirming.