"untruthful," early 13c., present-participle adjective from lie (v.1) "to tell an untruth." Related: Lyingly.
"untruthfulness, falsehood," c. 1300, "the telling of lies," verbal noun from lie (v.1) "to tell an untruth."
"an untruth; conscious and intentional falsehood, false statement made with intent to deceive," Middle English lie, from Old English lyge, lige "lie, falsehood," from Proto-Germanic *lugiz (source also of Old Norse lygi, Danish løgn, Old Frisian leyne (fem.), Dutch leugen (fem.), Old High German lugi, German Lüge, Gothic liugn "a lie"), from the root of lie (v.1).
To give the lie to "accuse directly of lying" is attested from 1590s. Lie-detector is recorded by 1909.
In mod. use, the word is normally a violent expression of moral reprobation, which in polite conversation tends to be avoided, the synonyms falsehood and untruth being often substituted as relatively euphemistic. [OED]
"given to lying, speaking falsely; having the characteristics of a lie, false, untrue," 1610s, from French mendacieux and directly from Latin mendacium "a lie, untruth, falsehood, fiction," from mendax (genitive mendacis) "lying, deceitful," from menda "fault, defect, carelessness in writing," from PIE root *mend- "physical defect, fault" (see amend (v.)). The sense evolution of Latin mendax was influenced by mentiri "to speak falsely, lie, deceive." Related: Mendaciously; mendaciousness.
"speak falsely, tell an untruth for the purpose of misleading," Middle English lien, from Old English legan, ligan, earlier leogan "deceive, belie, betray" (class II strong verb; past tense leag, past participle logen), from Proto-Germanic *leuganan (source also of Old Norse ljuga, Danish lyve, Old Frisian liaga, Old Saxon and Old High German liogan, German lügen, Gothic liugan), a word of uncertain etymology, with possible cognates in Old Church Slavonic lugati, Russian luigatĭ; not found in Latin, Greek, or Sanskrit. Emphatic lie through (one's) teeth is from 1940s.