We had a lively and ridiculous grammatical discussion at supper the other day.
This time of year everyone needs a new Ordo. It's a little book that helps you to find your way through the Liturgy of the Hours and the Eucharist each day. In our case, it also details the interaction of the general Roman calendar with our special Franciscan one, which has its own saints. For priests, it also offers suggestions for the prayers to choose for Mass, when there are options, of course.
Well, it turned out we needed two more than we had received. So our guardian was wondering how to ask for them. What is the plural? Is it Ordos, as we usually say in our sloppy speech? Or is it Ordoes, as our guardian opined, by analogy with potato?
Well, I said, it's obviously the Latin word ordo, ordinis that's at issue here, and since this is not a word that has "arrived" in English, we should form the plural in Latin.
But what is the use of ordo, ordinis in this case? Why is the book called the Ordo? The brothers assured me that it is short for Ordo Missae, "the order of Mass," but I don't buy this because the subtitle of the book is "The Order of Prayer in the Liturgy of the Hours and Celebration of the Eucharist."
My conclusion is that Ordo is short for Ordo Recitandi, "the order of reciting." Ok, that's all I need to know: Ordo is a simple noun.
Therefore, if we need more than one Ordo, we should ask for n Ordines. The full plural would be Ordines Recitandorum, wouldn't it? (someone with better Latin feel free to correct me.)