To be precise, it's when Celts speak sloppy Latin. Even the silly way they say 80 as 4x20 comes from the Gauls.

Know what ? Yeah that's the best description I've seen so far. So much for civilizing the Gaul

One plausible etymology is:

aqua > agua > egua > ewa > ewe > eawe > eaw > eau > o

So first the k sound changed to a g, then the initial a became an e, which lengthened to a diphthong, the final sound disappeared, the new final sound turned from a consonant into a vowel (like an oo) and the resulting triphthong reduced to a monophthong.

The elision of medial g is pretty surprising to me though.

here is a whole video on that exact topic

Aqua ➡️ eau is one of the explained examples 😁

I wonder when French took the wrong path,

When did Caesar invade Transalpine Gaul?

Germans are a lot worse by these standards ! "Poison" is "gift"

Just don't piscine the pool.

Edit: how can I forget good ol' toilet water! Eau de toilette