Etymology
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liar (n.)
"one who knowingly utters falsehoods," early 13c., from Old English leogere "liar, false witness, hypocrite," agent noun from Anglian legan, West Saxon leogan "be untruthful, lie" (see lie (v.1)). "The form in -ar is probably in imitation of the refashioned forms such as scholar for scoler and pillar for piler" [Barnhart]. A different formation yielded Dutch leugenaar, Old High German luginari, German Lügner, Danish lögner.
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Ananias 
"liar," a reference to Acts v.1-5.
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Cretan (n.)

Old English Cretense (plural), "natives or inhabitants of Crete, from Latin Cretanus (singular); see Crete. They were proverbial in ancient times as liars; compare Greek noun kretismos "lying," literally "Cretan behavior," and the classical sophism expressing the liar paradox (see one version below).  Alternative Cretic (c. 1600) was used especially of a form of verse.

Epimenides, the Cretan, says that all Cretans are liars.
If Epimenides' statement is not true, he is a liar; and if it is true, he is a liar, for he is a Cretan.
But his statement is either true or not true.
Therefore he is a liar.
But since he is a liar, his statement is not true that all Cretans are liars.
Therefore some Cretans are not liars.
But since some Cretans are not liars, Epimenides is not necessarily a liar because he is a Cretan.
Therefore, we may accept his statement that all Cretans are liars. And so on.
[John J. Toohey, "An Elementary Handbook of Logic," 1918]
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pathological (adj.)
1680s, "pertaining to disease," formed in English from pathologic + -al (1). Sense of "worthy to be a subject of pathology, morbid, excessive" (as in pathological liar) is attested from 1845. Related: Pathologically.
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mendacity (n.)

"tendency or disposition to lie, habitual lying," also "a falsehood, a lie," 1640s, from French mendacité and directly from Late Latin mendacitas "falsehood, mendacity," from Latin mendax "lying; a liar" (see mendacious).

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warlock (n.)
Old English wærloga "traitor, liar, enemy, devil," from wær "faith, fidelity; a compact, agreement, covenant," from Proto-Germanic *wera- (source also of Old High German wara "truth," Old Norse varar "solemn promise, vow"), from PIE root *were-o- "true, trustworthy." Second element is an agent noun related to leogan "to lie" (see lie (v.1); and compare Old English wordloga "deceiver, liar").

Original primary sense seems to have been "oath-breaker;" given special application to the devil (c. 1000), but also used of giants and cannibals. Meaning "one in league with the devil" is recorded from c. 1300. Ending in -ck (1680s) and meaning "male equivalent of a witch" (1560s) are from Scottish. Related: Warlockery.
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treacherous (adj.)
early 14c., from Old French trecheros, tricheros "deceitful" (12c.), from trecheor, tricheor "cheat, deceiver, liar, impostor, trickster," agent noun from trechier, trichier "to cheat, trick" (see trick (n.)). Figuratively, of things, from c. 1600. Related: Treacherously; treacherousness. Middle English had treacher "deceiver, cheat, traitor."
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barney (n.)

British slang word of uncertain origin, attested from 1859 as "a fixed or sham prize-fight," also "lark, spree, rough enjoyment;" 1864 as "noisy dispute."

"Notes and Queries," from March 21, 1863, describes Barnard Castle, the market town in Teesdale, as having "no enviable reputation. Longstaffe supposes that Sir George Bowes's refusal to fight with the rebels during the rising of the north, gave rise to the contemptuous distich:

'Coward, a coward of Barney Castell,
Dare not come out to fight a battel' "

And adds that "Come, come, that's a Barna' Cassell," is "a reproof to an exaggerator, or liar."

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gab (v.)
"talk much," 1786, probably via Scottish and northern England dialect from earlier sense "speak foolishly; talk indiscreetly" (late 14c.), from gabben "to scoff, jeer; mock (someone), ridicule; reproach (oneself)," also "to lie to" (late 13c.), from a Scandinavian source, such as Old Norse gabba "to mock, make fun of," and probably in part from Old French gaber "to mock, jest; brag, boast," which, too, is from Scandinavian. Ultimately perhaps imitative (compare gabble, which might have shaded the sense of this word). Gabber was Middle English for "liar, deceiver; mocker." Related: Gabbed; gabbing.
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Mephistopheles 

1590s, Mephastophilus, the name of the evil spirit to whom Faust sold his soul in the old legend, from German (1587), a word of unknown origin. The older, Greek-like form is apparently a folk-etymology. According to the speculation of eminent Göthe scholar K.J. Schröer (1886) it is a compound of Hebrew mephitz "destroyer" + tophel "liar" (short for tophel sheqer, literally "falsehood plasterer;" see Job xiii.4). Klein writes that the names of devils in the Middle Ages "are in most cases derived from Hebrew." Related: Mephistophelian.

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