Where does “dirtieth” come from?

dirtieth (English) comes from English dirty, from Middle English dirti, from Middle English dritti.

dirtieth (English): third-person singular simple present indicative...

Definitions

  1. third-person singular simple present indicative...

Ancestry of “dirtieth”, step by step

dirtieth traces back through two separate lines of ancestry.

via English dirty

StepLanguageWordMeaning
1EnglishdirtyUnclean; covered with or containing unpleasant...
2Middle Englishdirti—
3Middle Englishdritti—

via English eth

StepLanguageWordMeaning
1EnglishethA letter introduced into Old English to...
Every word from Middle English dritti →Every word from Middle English dirti →Every word from English dirty →