Where does “onything” come from?

onything (Middle English) comes from Old English aniþing, from Old English ǣniġ, from Proto-West Germanic *ainag — only, sole.

Ancestry of “onything”, step by step

StepLanguageWordMeaning
1Old Englishaniþing—
2Old Englishǣniġany
3Proto-West Germanic*ainagonly, sole
Every word from Proto-West Germanic *ainag →Every word from Old English ǣniġ →Every word from Old English aniþing →