The story
From un- + truthful. Compare Middle English untreuthfull, untrowþeful, wntreuthtfull (“unbelieving, infidel”).
First known use, by meaning
- from circa 1820s Not giving the truth; providing untrue facts; lying.
- from circa 1820s Pertaining to falsehood; corrupt; dishonest.
Etymology from Wiktionary, CC BY-SA 4.0.