Peter was just showing me pictures of the new Chevy Volt (which he likes, but really, it’s no 500), and I was musing on the cuteness of the name: “‘Volt’ is one of those stupid American car names that totally blends in with the background noise of stupid American car names, but then you think ‘oh I get it! Volt!’” And then I said, “‘Prius’? What is ‘Prius’, anyway? An alternate form of ‘Prig’?” So I Wiki “Prius” and oh look, a link in the contents: “Name“:

Name

The word prius is a Latin word meaning “[to go] before.”. A Toyota spokesman stated that “Toyota chose this name because the Prius vehicle is the predecessor of cars to come.”

Prius is not a verb but a Latin comparative adjective or adverb, the neuter nominative singular form of the adjective whose corresponding masculine and feminine nominative singular forms are prior.(see also Latin declension - Irregular adverbs and their comparative and superlative forms) with meanings “ahead, in front, leading; previous, earlier, preceding, prior; former; basic;”. Some enthusiasts enjoy using Prii as the plural, which would be correct if prius were a regular second declension Latin noun. The actual Latin plural of the adjective is priora. All of these forms are nominative case and there are several other forms for the other cases. As for the plural of Prius in English, Toyota has said that it is simply Prius and also that owners are welcome to use whatever they like.

God I love Wiki.