Au pied de la lettre

Literally: At the foot of the letter

Example: “Mon fils prend tout au pied de la lettre : je lui ai dit de sauter dans son pyjama, il a sauté.” — My son takes everything to the letter: I told him to jump into his pajamas, and he jumped.

English equivalent: To the letter

Origin: The letter is the written text as against its spirit, a distinction inherited from scriptural interpretation. Taking something au pied de la lettre often implies the reading is too narrow to be useful. French uses the shorter à la lettre in the same sense, and the two are interchangeable.