"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.
1868, adj. and noun, as a racial designation, literally "resembling the Mongols," from Mongol + -oid. Compare Mongolian. In reference to the genetic defect causing mental retardation (mongolism), by 1899, from the typical facial appearance of those who have it. See Down's Syndrome. Such people were called Mongolian from 1866.
"an error in action, opinion, or judgment," 1630s, from mistake (v.). The earlier noun was mistaking (c. 1300).
An error is a wandering from truth, primarily in impression, judgment, or calculation and, by extension of the idea, in conduct; it may be a state. A mistake is a false judgment or choice; it does not, as error sometimes does, imply moral obliquity, the defect being placed wholly in the wisdom of the actor, and in its treatment of this defect the word is altogether gentle. [Century Dictionary, 1897]
Meaning "unintended pregnancy" is from 1957. No mistake "no doubt" is by 1818.
"impairment, corruption," 1630s, from Latin vitiationem (nominative vitiatio) "violation, corruption," noun of action from past-participle stem of vitiare "to make faulty, injure, spoil, corrupt," from vitium "fault, defect, blemish, crime, vice" (see vice (n.1)).
1570s, "language, speech, mode of speech," especially "form of speech of a region or group, idiom of a locality or class" as distinguished from the general accepted literary language, also "one of a number of related modes of speech regarded as descended from a common origin," from French dialecte, from Latin dialectus "local language, way of speaking, conversation," from Greek dialektos "talk, conversation, speech;" also "the language of a country, dialect," from dialegesthai "converse with each other, discuss, argue," from dia "across, between" (see dia-) + legein "speak" from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')").