fifteoða [OE]
fiftenthe [MI]
fifteenth [ME]
fiftínþ
https://www.etymonline.com/word/fifteenth#etymonline_v_39701
https://www.merriam-webster.com/dictionary/fifteenth
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED15899
https://bosworthtoller.com/10686