Where does “Eau” come from?

Eau derives from Old English ēa, from Proto-Germanic ahwō, from Proto-Indo-European h₂ekʷeh₂, a root meaning water or aquatic feature.

Ancestry of “Eau”, step by step

Step	Language	Word	Meaning
1	Old English	ēa	flowing water, stream, river; water; river,...
2	Proto-West Germanic	ahu	river
3	Proto-Germanic	ahwō	water; stream, river
4	Proto-Indo-European	h₂ékʷeh₂	water

Eau — Etymology, Origin & Word History | Etymology Explorer