feowertigoða [OE]
fortithe [MI]
fortieth [ME]
fortieþ
https://www.etymonline.com/word/fortieth#etymonline_v_40752
https://www.merriam-webster.com/dictionary/fortieth
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED17496
https://bosworthtoller.com/10485