Avoir une langue de vipère

Literally: To have a viper's tongue

Example: “Méfie-toi d'elle, elle a une langue de vipère.” — Watch out for her; she has a poisonous tongue.

English equivalent: To have a sharp tongue

Origin: On record from 1643, in a poem by Paul Scarron, and probably older than that in speech. The viper supplies two things at once: real venom, which maps cleanly onto words designed to wound, and a long-standing cultural suspicion of the snake as devious and dangerous, reinforced in a Christian context by the serpent of Genesis standing in for the Devil himself. A viper met in a field was also a genuine, well-founded fear in the period the phrase took hold, which will have done nothing to soften the animal's reputation.