Uncertain. Traditionally a western Finnish dialectal word; no known cognates exist in other Finnic languages. Possibly borrowed from Proto-Germanic *aldaz ~ *alþijaz (“old”) (for the ending, compare tyyris < *diurijaz; for the gradation, compare raitis << *raidi(ja)z), but the semantics are unclear; a shift like “old, grown” → “mature” → “ready” could be supposed (perhaps with secondary influence by folk etymology with ala-, through the verb altistaa being associated with alistaa).