There is a document floating around that gives the history of the development of the car. In it they say that they named it that because their target was to introduce the car prior to the 21st century.
If anyone is interested, you can find that document (it's a book) about the history of the Prius in the following thread: http://priuschat.com/forums/prius-main-forum/45599-who-designed-prius.html
pri·us /ˈpraɪəs/ Pronunciation Key - Show Spelled Pronunciation[prahy-uhs] Pronunciation Key - Show IPA Pronunciation –adjective (in prescriptions) before; former. [Origin: 1890–95; < L]
[Origin: 1890–95; < L] wow I didnt realise the ancient times of latin speakers were only 118 years ago!
This brings up the other aspect of the name (which i'm sure has been asked many times): how is it pronounced? The listing above is definitely different from the more often heard Pree-uhs...
Yah know, it's never a good thing when the marketing department breaks out the Latin dictionary. Frankly, around here, we all cringe when we hear "it comes from Latin and it means...." At least in this case it worked out well....
so do you think that Ford might bring one out called Exodus - (which might be their last car made) or GM offers the Omegus (last word in gas guzzlers)... with apologies to all living Romans for the mis-spelling