eahtateoða [OE]
eighte-tenthe [MI]
eighteenth [ME]
étínþ
https://www.etymonline.com/word/eighteenth#etymonline_v_38011
https://www.merriam-webster.com/dictionary/eighteenth
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED13155
https://bosworthtoller.com/8371