mendacious adj.

early 13c., amenden, "to free from faults, rectify," from Old French amender "correct, set right, make better, improve" (12c.), from Latin emendare "to correct, free from fault," from ex "out" (see ex-) + menda, mendum "fault, physical blemish; error," from PIE *mend- "physical defect, fault" (source also of Sanskrit minda "physical blemish," Old Irish mennar "stain, blemish," Welsh mann "sign, mark;" Hittite mant- "something harming").

The spelling with a- is unusual but early and also is found in Provençal and Italian. In English, the word has been supplanted in senses of "repair; cure" by its shortened offspring mend (v.). The meaning "to add to legislation" (ostensibly to correct or improve it) is recorded from 1777. Related: Amended; amending.

"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).