Mendacious means dishonest or lying, especially in a deliberate, habitual way. A mendacious person twists the truth regularly and often skillfully, making deception a consistent pattern rather than an occasional mistake.

When a politician makes mendacious claims about an opponent, they know they're spreading falsehoods but do it anyway to gain an advantage. A mendacious witness in court tells lies under oath. A mendacious advertisement might promise results that a product simply cannot deliver.

The word carries more weight than just calling someone a liar. Mendacious suggests a pattern of deception, a kind of practiced dishonesty. It also sounds formal and serious because it often appears in news articles, legal documents, or academic writing rather than everyday conversation. You might say your friend told a lie, but you'd describe a corrupt official's testimony as mendacious.

The related noun is mendacity, which means the quality or practice of being dishonest. When people discuss the mendacity of a politician or a tabloid newspaper, they're talking about an established pattern of lying rather than a single mistake.