Wow! "pétrir" is directly from the Latin verb "pistrire". But in Latin, it means "to hit, to beat, to grind, to pound". In French, it is rather =knead (to "massage" the dough), as you said. And "to knead" in Latin is translated with fingere, malaxare. (So, the Spanish word, given by J.C.M.H, kept the Latin meaning).
Pistrire is very hard to find in a Latin dictionary, I don't know why, maybe it's a rare word. I've found it only in some French dictionaries giving the Latin etymology.
In the Lyonnais dialect (from the city of Lyon, France), there's another descend word. "pitrogner" meaning the act of kneading but in a very bad way, without savoir-faire.
Nominative of baker in Latin is Pistor for a man and Pistrix for a woman. Pistrix also means a whale, but I do not know why!
Here: http://www.dicolatin.com/FR/LAK/0/PISTRIX/index.htm ( in French)
Very interesting information, thanks. The amount we can learn from researching etymologies from Latin to Romance! It’s one of the best things about learning Latin, we improve our vocabularies in a Romance language, basically in every Romance language.
I just discovered that «pistrix» in that sense is from the greek πίστρις [pístris], which can mean “whale” as you say, and also any “sea monster”.: https://latinlexicon.org/definition.php?p1=2045386&p2=p
Virgil uses it in book 3 of the Aeneid:
«postrema immani corpore pistrix».
[In her lower parts she is a pistrix with a huge body].
A nice verb. maybe «fingo» will appear in the course at some point: «Ego fingo massam in pistrino», I knead dough in the bakery.
Interestingly, pistrino was a word for bakery in Italian, it comes from «pistrinum», which retains a part of the root pist as in «pistor», but again, they’re obsolete words.
What's cool though, is that the Latin verb that produced the word pistor «pinso», is actually where the Spanish verb pisar comes from. So from a morphological point of view, we can think of a «pistor» as a pisador :)
I admit I'm not a native English speaker, but "a bread" sounds like unnatural English to me. If you absolutely have to use an article, I would use "a loaf of bread". However, that would be a superfluous addition in this case as the original sentence makes no such specification.