Eighteenth

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Old English e(a)htatēne (see eight, -teen).

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eighteenth (adj., n.)

"next in order after the seventeenth; an ordinal numeral; being one of eighteen equal parts into which a whole is regarded as divided;" 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.