þritig [OE]
thritti [MI]
thirty [ME]
þerty
https://www.etymonline.com/word/thirty#etymonline_v_10763
https://www.merriam-webster.com/dictionary/thirty
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED45535
https://bosworthtoller.com/32120