I found my arms swathed down — my feet tied so fast that mine ankles ache at the very remembrance — the place was utterly dark — the oubliette, I suppose, of their accursed convent, and from the close, stifled, damp smell, I conceive it is also used for a place of sepulture.
— Ivanoe, Sir Walter Scott (1819)
This is a wonderful little word. An oubliette is a dungeon or prison cell whose only means of egress is through a trapdoor in the ceiling. For that reason, it’s usually deep underground, dark, cold, and made of earth and stone. It’s basically the opposite of the chamber in which Frodo was imprisoned in the Tower of Cirith Ungol, which was at the very top of the tower and could be reached only through a trapdoor in its floor, though I daresay one might still call that an oubliette.
The etymology of the word should be readily apparent to French speakers. It derives (fairly recently, too) from the French oublier “to forget”, which in turn comes from Latin oblīvisci “to forget” (and from which we derive the Modern English oblivion). With this etymology in mind, the chamber in the Tower of Cirith Ungol really would have become an oubliette if Sam hadn’t come along, and all the Orcs had killed each other off leaving Frodo all alone in the Tower!
With a slightly related etymology and meaning is perdition. Like oubliette, the word comes to us from Latin by way of French. In this case, it’s the French perdre “to lose” (which is not so different from forgetting), from Latin perdĕre, which means “to lose utterly; to destroy, finish, ruin” (more literally per “to an end” + dăre “to put”). Perdition refers more figuratively to eternal punishment in hell, which is rather what being lost or forgotten in an oubliette must feel like. I can’t help but think of the Man in the Iron Mask.
Are there opportunities to use oubliette metaphorically in the world today? Ever been stuck in an elevator between floors? I have, and I’d say it’s pretty close. A small, claustrophobic cell from which the only escape may be through a trapdoor in the ceiling. More rhetorically, one might use the word to describe a kind of figurative cul de sac in an argument, or perhaps a social or political trap into which one has fallen.