I wonder what Latin word or expression best describes a polymath, someone who masters several subjects. The two solutions I thought of are the following but I find them unsatisfactory.
The first one because it is simply a transliteration of the Greek and I am not able to find any Latin text using it. The second one because the meaning is wrong, I don't mean an omniscient man but simply one who has several areas of competence.
I do not have a complete and satisfying answer, but digging around revealed a couple of points of interest:
Johan Wower wrote De Polymathia tractatio: integri operis de studiis veterum in 1665. He uses the word polymathia throughout the text, but I did not spot a word for a polymath. The text is available in plain text, but the quality of the conversion is pretty low. He might have a suitable word somewhere in there.
A lot of online sources (including your question!) seem to give the Latin expression homo universalis, but I managed to find no Latin texts that use this expression. I am unsure whether it is simply a translation of the Italian 'uomo universale'. At any rate, this expression is a decent choice: easy to understand and clearly Latin.
The prefix poly- is Greek, and the corresponding Latin one would be multi-. This is very productive, as there are loads of Latin words starting with this. I would not even consider coining a new word with this prefix out of the question.
The L&S entry for multiplex gives examples like vir multiplex in virtutibus.
A good general word for (innate) ability is ingenium. Combining with my previous point, I think vir/femina/homo ingenii multiplicis would be a decent option.
In the medieval scholastic community one very early word for a polymath was doctor universalis and it was applied to Albertus Magnus and Alan of Lille. Later the Catholic church regularized the title and applied it to various scholars.
