"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.

"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).