thirteenth (adj., n.)
"next in order after the twelfth; an ordinal numeral; being one of twelve equal parts into which a whole is regarded as divided;" a 15c. metathesis of þriteenþe (mid-14c.; see thirteen + -th (1)), which replaced forms from Old English þreoteoða (West Saxon), þriteogeða (Anglian). Similar formation in Old Norse þrettande, Danish trettende, Swedish trettonde, Old Frisian threttinde, Dutch dertiende, Old High German dritto-zehanto, German dreizehnte.