Is it ensure, insure, or assure?
1620, "the action of throwing out of a window," from Latin fenestra "window" (see fenestration). A word invented for one incident: the "Defenestration of Prague," May 21, 1618, when two Catholic deputies to the Bohemian national assembly and a secretary were tossed out the window (into a moat) of the castle of Hradshin by Protestant radicals. It marked the start of the Thirty Years War. Some linguists link fenestra with Greek verb phainein "to show;" others see in it an Etruscan borrowing, based on the suffix -(s)tra, as in Latin loan-words aplustre "the carved stern of a ship with its ornaments," genista "the plant broom," lanista "trainer of gladiators." Related: Defenestrate (1915); defenestrated (1620).