There is a passage in the book Brush Up Your Mythology! by Michael Macrone that explains the concept of genius quite cleverly :
“It’s a good thing the Mensa Society didn’t have a chapter in ancient Rome, because there’s no way it could have kept its cherished exclusivity. Which is to say that there were a great many geniuses back then; in fact, while no man was one, every man had one.”
You see, in ancient Rome, a man’s spirit or his creative source was referred to as a genius.
Women had their own version of a genius. It was called a “juno”. Now, you may already recognize this name, since Juno was an important Roman goddess.
Incidentally for all of us who want to know our Latin, the plural of genius is genii.