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," inchoative verb from vanus "empty, void" (from PIE *wano-, suffixed form of root *eue- "to leave, abandon, give out"). Sense of "quickly vanishing, having no permanence" is by 1738.
