Etymology
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lacuna (n.)
"blank or missing portion in a manuscript," 1660s, from Latin lacuna "hole, pit," figuratively "a gap, void, want," diminutive of lacus "pond, lake; hollow, opening" (see lake (n.1)). The Latin plural is lacunae. The word has also been used in English from c. 1700 in the literal Latin sense in anatomy, zoology, botany. The adjectival forms have somewhat sorted themselves: Mathematics tends to use lacunary (1857), natural history lacunose (1816), and lacunar (n.) is used in architecture of paneled ceilings (1690s), so called for their sunken compartments. Leaving lacunal (1846) for the manuscript sense.
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*ghieh- 

Proto-Indo-European root meaning "to yawn, gape, be wide open." 

It forms all or part of: chaos; chasm; dehiscence; gap; gasp; gawp; hiatus; yawn.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vijihite "to gape, be ajar;" Greek khainein, Latin hiare "to yawn, gape;" Old Church Slavonic zinoti "to open (one's mouth);" Russian razinut', Serbo-Croatian zinuti, Lithuanian žioju, žioti, Czech zivati "to yawn;" Old English ginian, gionian "open the mouth wide, yawn, gape," Old Norse gina "to yawn," Dutch geeuwen, Old High German ginen "to be wide open," German gähnen "to yawn."

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interval (n.)
early 14c., "time elapsed between two actions or events," from Old French intervalle "interval, interim" (14c.), earlier entreval (13c.) and directly from Late Latin intervallum "a space between, an interval of time, a distance," originally "space between palisades or ramparts" [OED], from inter "between" (see inter-) + vallum "rampart, palisade, wall," which is apparently a collective form of vallus "stake," from PIE *walso- "a post" (see wall (n.)).

Metaphoric sense of "gap in time" also was in Latin. From c. 1400 in English as "a pause, an interruption in a state or activity." Musical sense "difference in pitch between two tones" is from c. 1600. Related: Intervallic.
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twain (n.)
Old English twegen "two" (masc. nominative and accusative), from Proto-Germanic *twa- "two," from PIE root *dwo- "two." It corresponds to Old Frisian twene, Dutch twee, Old High German zwene, Danish tvende. The word outlasted the breakdown of gender in Middle English and survived as a secondary form of two, especially in cases where the numeral follows a noun. Its continuation into modern times was aided by its use in KJV and the Marriage Service, in poetry (where it is a useful rhyme word), and in oral use where it is necessary to be clear that two and not to or too is meant. In U.S. nautical use as "a depth of two fathoms" from 1799.
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mayonnaise (n.)

sauce made from egg yolks and salad oil, beaten together with vinegar or lemon juice to the consistency of thickened cream and seasoned, 1815, from French sauce mayonnaise (1806), said by French sources to be corrupted from mahonnaise and to have been named in recognition of Mahon, seaport capital of island of Minorca, captured by France in 1756 after the defeat of the British defending fleet in the Seven Years' War; the sauce having been introduced either in commemoration of the victory, which was led by Armand de Vignerot du Plessis, duc de Richelieu (1696–1788), or because it was brought to France from there by him. But unless there is a gap in the record, the late date of appearance of the word make this seem doubtful. An inferior sort of Miracle Whip.

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lid (n.)

"movable or removable cover for a pot, etc.," mid-13c., from Old English hlid "covering, opening, gate," from Proto-Germanic *hlidan "a cover," literally "that which bends over" (source also of Old Norse hlið "gate, gap," Swedish lid "gate," Old French hlid, Middle Dutch lit, Dutch lid, Old High German hlit "lid, cover"), from PIE *klito-, from root *klei- "to lean."

Meaning "eyelid" is from early 13c. Slang sense of "hat, cap" is attested from 1896. As a measure of marijuana, one ounce, 1967, presumably the amount of dried weed that would fit in some commercial jar lid. Slang phrase put a lid on "clamp down on, silence, end" is from 1906; many figurative senses are from the image of a pot boiling over.

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bairn (n.)

"child" (of either gender or any age), "son or daughter," Old English bearn "child, son, descendant," from Proto-Germanic *barnan (source also of Old Saxon barn, Old Frisian bern, Old High German barn "child;" lost in modern German and Dutch), from PIE root *bher- (1) "to carry," also "to bear children."

Originally a general English word, in modern English restricted to northern England and Scottish from c. 1700. This was the English form of the original Germanic word for "child" (compare child). Dutch, Old High German kind, German Kind are from a prehistoric *gen-to-m "born," from the same root as Latin gignere (see genus and compare kind (n.)). Middle English had bairn-team "brood of children."

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politically (adv.)

late 15c., "according to fixed laws" (rather than unlimited power of a ruler); 1580s, "in a politic manner;" 1630s "in a political manner," from politic or political + -ly (2). The first sense is obsolete, the second rare or archaic. Politically correct is attested in prevailing current sense by 1970; abbreviation P.C. is from 1986.

[T]here is no doubt that political correctness refers to the political movement and phenomenon, which began in the USA, with the aim to enforce a set of ideologies and views on gender, race and other minorities. Political correctness refers to language and ideas that may cause offence to some identity groups like women and aims at giving preferential treatment to members of those social groups in schools and universities. [Thuy Nguyen, "Political Correctness in the English Language,"2007] 
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sound (n.1)

"noise, what is heard, sensation produced through the ear," late 13c., soun, from Old French son "sound, musical note, voice," from Latin sonus "sound, a noise," from PIE *swon-o-, from root *swen- "to sound."

The unetymological -d was established c. 1350-1550 as part of a tendency to add -d- after -n-. Compare gender (n.), thunder (n.), jaundice (n.), spindle, kindred, riband, and, from French powder (n.), meddle, tender (adj.), remainder, dialectal rundel, rundle for runnel, etc. First record of sound barrier is from 1939. Sound check is from 1977; sound effect is 1909, originally live accompaniment to silent films.

The experts of Victor ... will ... arrange for the synchronized orchestration and sound effects for this picture, in which airplane battles will have an important part. [Exhibitor's Herald & Moving Picture World, April 28, 1928]
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breach (n.)

Old English bryce "a fracture, act of breaking," from Proto-Germanic *brukiz (source also of Old Frisian breke "a burst, crack, demolition (of a house)," Old Saxon bruki, Old High German bruh, Middle Dutch broke), a noun from *brekanan (source of Old English  brecan "to shatter, burst; injure, violate, destroy, curtail;" see break (v.)). The English word was influenced by Old French cognate breche "breach, opening, gap," which is from Frankish or another Germanic source. Ultimately from PIE root *bhreg- "to break."

Figurative sense of "infraction, violation, a breaking of rules, etc." was in Old English. Meaning "opening made by breaking" is from late 14c. Meaning "rupture of friendly relations" is from 1570s. Breach of contract is from at least 1660s; breach of peace "violation of public order" is from 1670s; breach of promise (usually promise of marriage) is from 1580s.

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