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Mendacious
Negative Connotation
Mendacious has a strong negative feeling, suggesting intentional dishonesty, not just a small or harmless lie. 
Formal Language
'Mendacious' is formal and might not be used in casual conversation; it’s more common in written English or formal speech. 
Cultural Usage
Mendacious may appear in contexts discussing ethics or philosophy, where truth and deception are central themes. 