vainyoudaoicibaDictYouDict[vain 词源字典]
vain: [13] Latin vānus meant ‘empty’ (it was related to vacuus ‘empty’, source of English vacuum). It passed into English via Old French vain in the sense ‘worthless’, and the main modern meaning ‘conceited’ did not develop until the 17th century. Also from vānus come English evanescent, vanish, vanity [13], and vaunt [14], and wane, want, etc go back to the same ultimate Indo-European base.
=> evanescent, vanish, vanity, vaunt, wane, want[vain etymology, vain origin, 英语词源]
vain (adj.)youdaoicibaDictYouDict
c. 1300, "devoid of real value, idle, unprofitable," from Old French vain, vein "worthless, void, invalid, feeble; conceited" (12c.), from Latin vanus "empty, void," figuratively "idle, fruitless," from PIE *wa-no-, from root *eue- "to leave, abandon, give out" (cognates: 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," Armenian unain "empty").

Meaning "conceited, elated with a high opinion of oneself" first recorded 1690s in English; earlier "silly, idle, foolish" (late 14c.). Phrase in vain "to no effect" (c. 1300, after Latin in vanum) preserves the original sense. Related: Vainly; vainness. Compare also vainglory.