Where does “þridda” come from?
þridda (Old English) comes from Proto-West Germanic *þriddjō, from Proto-Germanic þridjô, from Proto-Indo-European tretyós, from Proto-Indo-European tr̥tyós, from Proto-Indo-European tréyes — three.
þridda (Old English): third
Ancestry of “þridda”, step by step
Words derived from “þridda”