fifteenth (adj., n.)
"next in order after the fourteenth; an ordinal numeral; being one of fifteen equal parts into which a whole is regarded as divided;" late 14c., from fifteen + -th (1). By 15c. displacing forms derived from Old English fifteoða. Compare Old Frisian fiftuda, Dutch vijftiende, German fünfzehnte, Old Norse fimmtandi, Gothic fimftataihunda, with ordinal -d where English has -th. As a noun by late 14c.