Where does “ath” come from?

ath (Middle English) comes from Old English āþ, from Proto-West Germanic aiþ, from Proto-Germanic aiþaz, from Proto-Indo-European oyt-.

Ancestry of “ath”, step by step

StepLanguageWordMeaning
1Old Englishāþoath
2Proto-West Germanicaiþoath
3Proto-Germanicaiþazoath
4Proto-Indo-Europeanoyt-—
Every word from Proto-Indo-European oyt- →Every word from Proto-Germanic aiþaz →Every word from Proto-West Germanic aiþ →