I've fallen a little behind with this blog due to traveling over Thanksgiving, but on the plus side, a couple of things came up which provide fuel for some new posts. I'll see what I manage to get to, but the first one was when my sister wondered aloud where the word facetious came from. And it did seem curious. You'd have to guess that it relates to "facet" in some way, but it is hard to make the link between facet, which must have to do with an aspect or surface of something, and an adjective that means something along the lines of 'said in jest'. Isn't it?
Hmm. No relation. They come to us through the same languages, but have different roots. Facetious goes back to the French word facétieux with roughly the same meaning, and stems from facétie, a joke. The Latin facetia was a jest or witticism and came from facetus, which could mean witty, but also meant elegant, fine or courteous.
Facet, on the other hand, comes from the French facette, a diminutive for face. The Online Etymology Dictionary tells us that the usage in diamond cutting is in fact the original usage.
I was a bit surprised, though, that to be facetious seems to have taken on a more negative meaning than I really associate with it. Many people seem to think it means to jest in a heavy-handed or ill-timed way. I actually found a long discussion of the difference between being facetious and being sarcastic here. Suffice to say that commenters had many and conflicting thoughts on this. I would say that facetious has become a bit, well, multifaceted in its interpretation.
Relating facetious to facet still doesn't seem all that far-fetched a guess to me. At least, it seems more likely than a question I saw in the midst of my research, which asked if facetious is related to feces. Although my initial reaction was "how did you come up with that?" there may be something in this. It turns out that there is a rare book from 1470 called The Facetiae, which is a collection of jokes, many of which are scatological. They were assembled by one Gian Francesco Poggio Bracciolini. A scholar and a humanist, he doesn't look like a facetious type--at least if his portrait on Wikipedia is anything to go by...