eighteenth (adj.)youdaoicibaDictYouDict[eighteenth 词源字典]
mid-13c., egtetenþe, modified, by influence of eighteen, from Old English eahtateoða; from eight + teoða "tenth" (see -ty (1)). Cognate with German achtzehnte, Danish attende, Swedish adertonde.[eighteenth etymology, eighteenth origin, 英语词源]