Mendacious
Etymology
From Latin mendax, "lying, false," connected to mendum, "a fault or defect." The link is telling: a lie was seen as a flaw or blemish in what should be true, and the word carries that sense of a defect in the telling.
adjective · /men-DAY-shus/
Dishonest; given to lying.
What it names
A formal, damning word for habitual untruthfulness — stronger and more clinical than "lying."
Example
The report was so mendacious that even its footnotes couldn’t be trusted.
Related idea
Deceitful stresses the intent to trick; mendacious stresses the falsehood itself.