mendax

Etymology
Derived from, whence also and , plus. Original meaning was "faulty" and was shifted towards "deceitful" under the influence of.

Adjective

 * 1) deceitful, lying, deceptive, untruthful, false, mendacious, feigned, not real.

Declension
Note that mendāce is an alternative form for the ablative singular mendācī

Noun

 * 1) liar