Is it ensure, insure, or assure?
early 14c., "free from spiritual pollution," from Old French purefier "purify, cleanse, refine" (12c.), from Latin purificare "to make pure," from purus "pure" (see pure) + root of facere "to make" (see factitious). Meaning "free from extraneous matter" is recorded from mid-15c. Related: Purified; purifying.