Etymology
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fourth estate (n.)

"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]
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estate (n.)

early 13c., "rank, standing, condition," from Anglo-French astat, Old French estat "state, position, condition, health, status, legal estate" (13c., Modern French état), from Latin status "state or condition, position, place; social position of the aristocracy," from PIE root *sta- "to stand, make or be firm."

For the unetymological e-, see e-. Sense of "property" is late 14c., from that of "worldly prosperity;" specific application to "landed property" (usually of large extent) is first recorded in American English 1620s. A native word for this was Middle English ethel (Old English æðel) "ancestral land or estate, patrimony." Meaning "collective assets of a dead person or debtor" is from 1830.

The three estates (in Sweden and Aragon, four) conceived as orders in the body politic date from late 14c. In France, they are the clergy, nobles, and townsmen; in England, originally the clergy, barons, and commons, later Lords Spiritual, Lords Temporal, and commons. For Fourth Estate see four.

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fourth (adj., n.)

"next in order after the third; an ordinal numeral; being one of four equal parts into which a whole is regarded as divided;" mid-15c., alteration (by influence of four), of ferthe, from Old English feorða "fourth," from Proto-Germanic *feurthan (source also of Old Saxon fiortho, Old Norse fiorðe, Dutch vierde, Old High German fiordo, German vierte); see four + -th (1). As a noun from 1590s, both of fractions and in music.

Among the old Quakers, who rejected the pagan weekday names, fourth day was Wednesday, often a secondary day of meeting for worship. Fourth-dimension attested from 1844. The theatrical fourth wall is from 1807. The celebration of the Fourth of July as the epoch of American independence is attested from 1777.

That there is due to Daniel Smith, of the city tavern, for his bill of expences of Congress, on the 4 of July last, including a balance of an old account, the sum of 729 68/90 dollars; also a bill for materials, workmanship, &c furnished for the fire works on the 4 July, the sum of 102 69/90 dollars .... [Auditor General's report, Aug. 8, 1777, Journals of Congress, vol. VII]
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quart (n.)

liquid measure of capacity equal to one-fourth of a gallon, early 14c., from Old French quarte "a fourth part" (13c.), from Latin quarta (pars), fem. of quartus "the fourth, fourth part" (related to quattuor "four," from PIE root *kwetwer- "four").

Compare Latin quartarius "fourth part," also the name of a small liquid measure (the fourth part of a sextarius), which was about the same as an English pint.

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firkin (n.)

"small cask," late 14c., apparently from Middle Dutch *vierdekijn, diminutive of vierde, literally "fourth, fourth part" (from vier "four," from PIE root *kwetwer- "four"); so called because it usually is the fourth part of a barrel.

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quartan (adj.)

"having to do with the fourth," especially of attacks of an intermittent fever, etc., "occurring every fourth day" (by inclusive reckoning; now we would say every third day), early 14c., (feuer) quartain, from Old French (fievre) quartaine or cartaine and directly from Latin (febris) quartana, "quartan (fever)," fem. of quartanus "of or belonging to the fourth; of or occurring on the fourth day," from quartus "the fourth, fourth part" (related to quattuor "four," from PIE root *kwetwer- "four").

Also as a noun, "an ague or fever which recurs on the fourth (third) day," late 14c. Under inclusive reckoning, both days of consecutive occurrence are counted (if you have it on Wednesday and again on Saturday, the ancients would count that as "every four days"). 

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quartet (n.)

also quartette, 1773, "musical composition for four solo instruments or voices," from French quartette, from Italian quartetto, diminutive of quarto "fourth," from Latin quartus "the fourth, fourth part" (related to quattuor "four," from PIE root *kwetwer- "four"). Meaning "set of four singers or musical players who perform quartets" is from 1814.

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quarto (n.)

"book from paper folded twice to make four pages to the sheet, size of book in which the leaf is one-fourth of a certain size of paper," late 15c., in the phrase in quarto, from Medieval Latin in quarto "in the fourth (part of a sheet of paper)," from quarto, ablative singular of Latin quartus "the fourth, fourth part" (related to quattuor "four," from PIE root *kwetwer- "four").

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quadroon (n.)

by 1781, an alteration (by influence of words in quadr-) of quarteroon (1707), "offspring of a white and a mulatto," from Spanish cuarteron (used chiefly of the offspring of a European and a mestizo), literally "one who has a fourth" (Negro blood), from cuarto "fourth," from Latin quartus "the fourth, fourth part," which is related to quattuor "four" (from PIE root *kwetwer- "four").

So called because he or she has one quarter African blood. There also was some use in 19c. of quintroon (from Spanish quinteron) "one who is fifth in descent from a Negro; one who has one-sixteenth Negro blood." OED lists quarter-caste as an Australian and New Zealand term for a person whose ancestry is one-quarter Aboriginal or Maori and 3/4 white (1948).

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hemidemisemiquaver (n.)

"sixty-fourth note" in music, 1846, from hemi- + demi- + semi- + quaver (n.).

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