1717, "on the point of becoming imperceptible," from French évanescent, from Latin evanescentem (nominative evanescens), present participle of evanescere "disappear, vanish, pass away," figuratively "be forgotten, be wasted," from assimilated form of ex- "out" (see ex-) + vanescere "vanish" (see vanish). Sense of "quickly vanishing, having no permanence" is by 1738.