Etymology
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fault (n.)

late 13c., faute, "deficiency," from Old French faute, earlier falte, "opening, gap; failure, flaw, blemish; lack, deficiency" (12c.), from Vulgar Latin *fallita "a shortcoming, falling," from Latin falsus "deceptive, feigned, spurious," past participle of fallere "deceive, disappoint" (see fail (v.)).

The -l- was restored 16c., probably in imitation of Latin, but the letter was silent until 18c. Sense of "physical defect" is from early 14c.; that of "moral culpability" (milder than sin or vice, but more serious than an error) is first recorded late 14c. Geological sense is from 1796. The use in tennis (c. 1600) is closer to the etymological sense.

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Schrödinger's cat 

by 1972 in reference to the thought experiment proposed in 1935 by Austrian-born physicist Erwin Schrödinger (1887-1961) in correspondence with Albert Einstein about quantum mechanics. Schrödinger was pointing out a problem in the then-prevailing interpretation of the science. Einstein had written, as an example, about an unstable keg of gunpowder that will in some quantum sense exist in both exploded and unexploded states. To point out the flaw Schrödinger wrote "One can even set up quite ridiculous cases," and described the cat situation, in which the animal is both dead and alive until its state has been observed.

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pop (adj.)

"having popular appeal," 1926, of individual songs from many genres; 1954 as a noun, as genre of its own; abbreviation of popular; earlier as a shortened form of popular concert (1862), and often in the plural form pops. Pop art is recorded from 1957, said to have been in use conversationally among Independent group of artists from late 1954. Pop culture attested from 1958, short for popular culture (which is attested by 1846).

To dismiss him [Johnnie Ray] out of hand one would have to share (as I can't) that facile contempt for "pop" culture, and by implication "pop" audiences, which is the principal flaw of that ambitious new musical, "Expresso Bongo." [Kenneth Tynan, "At the Theatre," The Observer, May 11, 1958]
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defective (adj.)
Origin and meaning of defective

mid-14c., "having a defect or flaw of any kind, inferior, in bad condition," from Old French défectif (14c.) and directly from Late Latin defectivus "imperfect," from defect-, past-participle stem of deficere "to desert, revolt, fail," from de "down, away" (see de-) + combining form of facere "to do, make" (from PIE root *dhe- "to set, put").

[D]efective generally takes the sense of lacking some important or essential quality; deficient, that of lacking in quantity: as defective teeth, timber, character; deficient supplies, means, intellect. The same difference is found between deficiency and defectiveness. [Century Dictionary]

In grammar, "wanting some of the usual forms of declension or conjugation" (late 15c.). Used in the sense of "mentally ill" from 1854 to c. 1935; as a noun, "person wanting in some physical or mental power," by 1869. Related: Defectively; defectiveness.

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error (n.)

also, through 18c., errour; c. 1300, "a deviation from truth made through ignorance or inadvertence, a mistake," also "offense against morality or justice; transgression, wrong-doing, sin;" from Old French error "mistake, flaw, defect, heresy," from Latin errorem (nominative error) "a wandering, straying, a going astray; meandering; doubt, uncertainty;" also "a figurative going astray, mistake," from errare "to wander; to err" (see err). From early 14c. as "state of believing or practicing what is false or heretical; false opinion or belief, heresy." From late 14c. as "deviation from what is normal; abnormality, aberration." From 1726 as "difference between observed value and true value."

Words for "error" in most Indo-European languages originally meant "wander, go astray" (for example Greek plane in the New Testament, Old Norse villa, Lithuanian klaida, Sanskrit bhrama-), but Irish has dearmad "error," from dermat "a forgetting."

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pleonasm (n.)

"redundancy in words," 1580s, from Late Latin pleonasmus, from Greek pleonasmos, from pleonazein "to be more than enough, to be superfluous," in grammatical use, "to add superfluously," from combining form of pleon "more" (from PIE root *pele- (1) "to fill"). Not necessarily a flaw in rhetoric and sometimes used effectively for emphasis. As Fowler writes, "The writer who uses [pleonasm] in that way must be judged by whether he does produce his effect & whether the occasion is worthy of it."

The first surplusage the Greekes call Pleonasmus, I call him [too full speech] and is no great fault, as if one should say, I heard it with mine eares, and saw it with mine eyes, as if a man could heare with his heeles, or see with his nose. We our selues ysed this superfluous speech in a verse written of our mistresse, neuertheles, not much to be misliked, for euen a vice sometime being seasonably vsed, hath a pretie grace. [George Puttenham, "The Arte of English Poesie, 1589]
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