oubliette (n.)

late 14c., oblivioun, "state or fact of forgetting, forgetfulness, loss of memory," from Old French oblivion (13c.) and directly from Latin oblivionem (nominative oblivio) "forgetfulness; a being forgotten," from oblivisci (past participle oblitus) "forget," which is of uncertain origin.

Perhaps originally "even out, smooth over, efface," from ob "over" (see ob-) + root of lēvis "smooth," but de Vaan and others find that "a semantic shift from 'to be smooth' to 'to forget' is not very convincing." However no better explanation has emerged. Latin lēvis also meant "rubbed smooth, ground down," from PIE *lehiu-, from root *(s)lei- "slime, slimy, sticky" (see slime (n.)); for sense evolution, compare obliterate.

Meaning "state or condition of being forgotten or lost to memory" is from early 15c. In English history, the Acts of Oblivion use the word in the sense of "intentional overlooking" (1610s), especially of political offenses. Related: Obliviously; obliviousness.

Oblivion is the state into which a thing passes when it is thoroughly and finally forgotten. ... Forgetfulness is a quality of a person: as a man remarkable for his forgetfulness. ... Obliviousness stands for a sort of negative act, a complete failure to remember: as a person's obliviousness of the proprieties of an occasion. [Century Dictionary]