sixtigoþa [OE]
sixtithe [MI]
sixtieth [ME]
sixtieþ
https://www.etymonline.com/word/sixtieth#etymonline_v_48704
https://www.merriam-webster.com/dictionary/sixtieth
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED40602
https://bosworthtoller.com/27907