*Masō

From a Celtic language, presumably Proto-Celtic *Mosā. Despite the obvious similarities, the name is not derived from Latin Mosa. It shows evidence of the change from of o to a; this is known to have occurred before the first contact of Germanic people with the Romans, but probably after first contact with the Celts. Most likely, Latin and Germanic both borrowed the name from Celtic independently.

Albrecht Greule writes that its ultimate origin is unclear, but could perhaps be Proto-Indo-European *meh₂d-, whence Latin madeō (‘I am wet’) and Ancient Greek μεστός (mestós) (mestós, ‘full’). Or, from Proto-Indo-European *mā- (‘to stupefy’) in the sense of the river's tortuousness, cognate with Proto-Germanic *masōną (‘to confound, be weary, dream’) (modern English maze), Welsh mydu (‘to vault, arch’), Old Norse meis (‘curvatura’).