Is it ensure, insure, or assure?
early 14c., "lose brightness, grow pale," from Old French fader "become weak, wilt, wither," from adj. fade "pale, weak, insipid" (12c.), probably from Vulgar Latin *fatidus, some sort of blending of Latin fatuus "silly, tasteless" + vapidus "flat, flavorless." Related: Faded; fading. As a noun, from c.1300.