1620, "the action of throwing out of a window," from Latin fenestra "window." 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 of the castle of Hradschin by Protestant radicals (the pair landed in a trash heap and survived). It marked the start of the Thirty Years' War.
The extraordinary chance which had saved three lives was a holy miracle or a comic accident according to the religion of the beholder .... Murder or no murder, the coup d'état was complete, and since Thurn had overruled many of his supporters in demanding death it was well for the conscience of his allies that a pile of mouldering filth in the courtyard of the Hradschin had made soft falling for the governors. [Cicely Veronica Wedgwood, "The Thirty Years War," 1938]
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).
1870 in the anatomical sense, noun of action from Latin fenestrare, from fenestra "window, opening for light," a word perhaps from Etruscan (see defenestration). Meaning "arrangement of windows" as a design element in architecture is from 1846. Related: Fenestrated.