c.1300, "devoid of real value, idle, unprofitable," from Old French vein "worthless," from Latin vanus "idle, empty," from PIE *wa-no-, from root *eue- "to leave, abandon, give out" (cf. Old English wanian "to lessen," wan "deficient;" Old Norse vanta "to lack;" Latin vacare "to be empty," vastus "empty, waste;" Avestan va- "lack," Persian vang "empty, poor;" Sanskrit una- "deficient"). Meaning "conceited" first recorded 1690s, from earlier sense of "silly, idle, foolish" (late 14c.). Phrase in vain "to no effect" (c.1300, after Latin in vanum) preserves the original sense. Related: Vainly.
