"excuse for being," 1864, first recorded in letter of J.S. Mill, from French raison d'être, literally "rational grounds for existence."
c. 1600, "victory involving one's own ruin," translating Greek Kadmeia nikē, from Cadmus (Greek Kadmos), legendary hero-founder of Thebes in Boeotia and bringer of the original sixteen-letter alphabet to Greece. Probably a reference to the story of Cadmus and the "Sown-Men," who fought each other till only a handful were left alive. Compare Pyrrhic (adj.1).
1650s, "public department in charge of letter-carrying," from post (n.3) + office. Meaning "building where postal business is carried on, office or place where letters are received for transmission," is from 1650s. In slang or euphemistic sense of "a sexual game" it refers to an actual parlor game first attested early 1850s in which pretend "letters" were paid for by kisses.
c. 1300, Prestre Johan, legendary medieval Christian king and priest, said to have ruled either in the Far East or Ethiopia. Prestre (attested as a surname late 12c.) is from Vulgar Latin *prester, a transition between Latin presbyter and English priest. First mentioned in the West by mid-12c. chronicler Otto of Freising, who told how Johannes Presbyter won a great victory over the Persians and the Medes. Between 1165 and 1177 a forged letter purporting to be from him circulated in Europe. All this recalls the time when the Christian West was militarily threatened on its frontiers by Muslim powers, dreaming of a mythical deliverer. Compare Old French prestre Jehan (13c.), Italian prete Gianni.
"human being eaten as food," by 1848, in stories from the Fiji Islands, said to be a literal rendering of a local term, in one version puaka balava.
Bau literally stank for many days, human flesh having been cooked in every house, and the entrails thrown outside as food for pigs, or left to putrefy in the sun. The Somosomo people were fed with human flesh during their stay at Bau, they being on a visit at that time; and some of the Chiefs of other towns, when bringing their food, carried a cooked human being on one shoulder, and a pig on the other; but they always preferred the "long pig," as they call a man when baked. ["FEEJEE.—Extract of a Letter from the Rev. John Watsford, dated Ono, October 6th, 1846." in "Wesleyan Missionary Notices," Sept. 1847]