In lexicons of sea language going back to 1759, the bitter end is the part of a cable which is round about the bitts (the two great timbers used to belay cables) when the ship is at anchor (see bitt).
Bitter end of the Cable, the End which is wound about the Bitts. ["The News-Readers Pocket-Book: Or, a Military Dictionary," London, 1759]
So, when a cable is played out to the bitter end, there is no more left to play. The term began to be used c. 1835 in non-nautical use and with probable influence of or merger with bitter (adj.).
"the press," by 1824, and especially from 1831, British English. For the other three, see estate. Earlier the term had been applied in various senses that did not stick, including "the mob" (1752), "the lawyers" (1825). The extension to the press is perhaps an outgrowth of the former.
Hence, through the light of letters and the liberty of the press, public opinion has risen to the rank of a fourth estate in our constitution; in times of quiet and order, silent and still, but in the collisions of the different branches of our government, deciding as an umpire with unbounded authority. ["Memoir of James Currie, M.D.," 1831]
[Newspapers] began to assume some degree of political importance, during the civil wars of the seventeenth century, in England; but it is not until within the last fifty years that they have become, — as they are now justly styled, — a Fourth Estate, exercising a more powerful influence on the public affairs of the countries in which they are permitted to circulate freely, than the other three put together. [Alexander H. Everett, "Address to the Phi Beta Kappa Society of Bowdoin College," 1834]
The new and exquisitely beautiful theatre of Drury-lane has the peculiar contrivance of an iron-curtain to secure the audience from all danger, in case of fire on the stage. Miss Farren, in the occasional epilogue, delivered on opening this new theatre, pleasantly informs the spectators that, should flames burst out in the part appropriated to the representation, they may comfort themselves with thinking that nothing can be burnt but the scenery and the actors. [The Monthly Review, June 1794]
From 1819 in the figurative sense "impenetrable barrier." In reference to the Soviet occupation of Eastern Europe, famously coined by Winston Churchill March 5, 1946, in speech at Westminster College, Fulton, Missouri, but it had been used earlier in this context (for example by U.S. bureaucrat Allen W. Dulles at a meeting of the Council on Foreign Relations, Dec. 3, 1945). The phrase had been used in the sense of "barrier at the edge of the sphere of influence of the Soviet Union" from 1920. During World War II, Goebbels used it in German (ein eiserner Vorhang) in the same sense. But its popular use in the U.S. dates from Churchill's speech.