A liar is always lavish of oaths.

The aphorism links habitual lying with an overuse of solemn assurances: the liar, lacking credibility, compensates by multiplying oaths, vows, and emphatic guarantees. Corneille’s line reflects a classical moral psychology common in seventeenth-century French literature, where speech is a marker of character and honor. The observation also implies a practical rule of judgment: the more someone insists on their truthfulness through swearing, the more reason there may be to doubt them. In this sense, the quote critiques performative sincerity—language that tries to manufacture trust rather than earn it through consistent truth-telling.