Ahti.

Probably ahtaa + -i. The consonantism implies a relatively recent derivation, but based on the dialectal meaning "to hang up to dry". The god of water and fishing would have thus originally been a god that helps keep fish nets dry and thus by extension bring a good catch. Alternatively possibly borrowed from Proto-Samic *āččē (“father”), an epithet that occurs in many names of Sami deities.

A confluence of multiple different etymologies. Partly from the mythological name (see Ahti, etymology 1), partly from Proto-Norse [Term?] (“wealth, property”) (as the name of a mythical warrior) and partly from the Middle Low German given name Ahti, which in turn is ultimately from Ancient Greek αὐτόνομος (autónomos). The Russian name Автоном (Avtonom) (from the same Ancient Greek source) may be a fourth source (at least in eastern Finland). Mikkonen & Paikkala (2000) considers the Germanic name to be the most likely source, while the Russian source might be relevant only for the eastern Finnish occurrences of this name. The given name first came into use as an official given name in 1879.