Where does “trenything” come from?

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

Ancestry of “trenything”, step by step

trenything traces back through two separate lines of ancestry.

via English anything

StepLanguageWordMeaning
1EnglishanythingAny object, act, state, event, or fact whatever;...
2Middle Englishanything
3Old Englishaniþing
4Old Englishǣniġany
5Proto-West Germanic*ainagonly, sole

via English tren

StepLanguageWordMeaning
1EnglishtrenA fish spear
Every word from Proto-West Germanic *ainagEvery word from Old English ǣniġEvery word from Old English aniþing