Is it ensure, insure, or assure?
1540s, "practicing illusion or magic, deceptive," from Latin praestigious "full of tricks," from praestigiae "juggler's tricks," probably altered by dissimilation from praestrigiae, from praestringere "to blind, blindfold, dazzle," from prae "before" (see pre-) + stringere "to tie or bind" (see strain (v.)). Derogatory until 19c.; meaning "having dazzling influence" is attested from 1913 (see prestige). Related: Prestigiously; prestigiousness.