Mendacity means untruthfulness or the habit of lying. It's a formal word for dishonesty, especially when someone lies repeatedly or makes lying part of how they operate.

When a politician is accused of mendacity, people are saying he doesn't just tell an occasional fib but regularly twists the truth to serve his purposes. When a witness in court shows mendacity, she's caught in multiple lies that undermine everything she says. The word suggests something deeper than a single mistake or white lie: it points to a pattern of deception.

You might hear mendacity discussed in serious contexts like news reports, history books, or courtrooms. A historian might write about the mendacity of a dictator's propaganda machine. A lawyer might expose the mendacity of someone who keeps changing their story.

The related adjective is mendacious, which describes someone or something characterized by lies. A mendacious account of events would be one that's deliberately misleading.

While “liar” and “lying” are everyday words kids use naturally, mendacity is the kind of word you'd encounter in more formal writing or grown-up conversation. It carries extra weight because of its seriousness and formality, making it useful when you want to emphasize just how dishonest someone or something really is.