The Latin wisdom of crowds prevails... barely! Toyota hosted an online poll to ask people what they think the plural of Prius should be. By a narrow one-point plurality, respondents chose the Latin form Prii. Toyota says that is the form it will use in its ads for the flagship of the hybrid industry.
Now Prius isn't really Latin, so purists may rankle at this weird splicing of historical linguistics. But I embrace this classical pluralization just as I embrace the amusing and widespread pluralization that take place in high school debate, where we refer to hordes of giggly, gangly novice debaters as novi. It just sounds right.
Back in the automotive world, can the Ford Focus be far behind? Remember, the c goes soft in Foci, just like in geometry.
I can see a whole panoply of peppy Prius pitches popping from this pleasant plural. Imagine sweeping shots of Prii, cruising by cacti, humming past hippopotami, and demonstrating their tight turning radii. Toyota could tour America, visiting Georgia, Indiana, Mississippi, Wisconsin, and Ohio to show how easily Prii can get around Columbi.
Surely conservatives are rankling at all this free press for Toyota's hybrid car. After all, the only form of transportation conservatives hate more than Prii are all those darned collectivist bi.
Renovatio latinae!
In classical Latin, isn't the C always hard, like our K? Of course, the "I" would also be pronounced "ee," so Foci would be "fokee" -- but then, there aren't any ancient Romans around to set us straight, so we can do what we want.
BTW, another plural: Cincinnati is the pluralized name of the archetypal citizen-soldier Cincinnatus, isn't it?
My analytical geometry teacher, Joe Austin, may have been around during Roman times, and he consistently said /FO - si/, long i.
Your teacher was probably right. All I know is what I read in a Latin textbook. If I recall, choral pronunciation of musical works in Latin varies in still other ways.