Is it ensure, insure, or assure?
late 14c., from Old French amortiss-, present participle stem of amortir "deaden," from Vulgar Latin *admortire "to extinguish," from ad- "to" (see ad-) + mortus "dead," from Latin mors "death" (see mortal (adj.)). Originally a legal term for an act of alienating lands. Meaning "extinguish a debt" (in form amortization) is attested from 1824. Related: Amortized; amortizing.