The other day I found some stuff I'd written inside the cover of a book years ago and had totally forgotten. It's the origins of do re mi (originally ut re mi) from a hymn by Guido d'Arezzo. You might find it interesting to read this wikipedia page on it.
It's a little more complicated than that - originally the scale began on A in the bass (and only went up as far as F, hence hexachord), then was lowered to G (known as gamma ut, whence the word gamut), and so on. I found it by accident originally in the dictionary under "Aretinian" (adjective pertaining to Guido d'Arezzo), probably during a failed search for Aretalogy, lol!
Here's a summary of the Wiki page that I've copied from an old blog
Ut queant laxis
"Ut" was changed in the 1600s in Italy to Do, at the suggestion of the musicologist Giovanni Battista Doni (based on the first syllable of his surname). In Anglophone countries, "Si" was changed to "Ti" by Sarah Glover in the nineteenth century so that every syllable might begin with a different letter.
The original Ut was the note A. When G below it was added, it was called GammaUt, which gives us the word "gamut".
Nowadays Do always means "C" (e.g. "Fa dièse" is French for F#), except in countries where it just means the same as "tonic" (a friend went on holiday to Turkey once and met some musicians and asked what notes they tuned their instruments to. All he could get from them was "Do")