À l'œil

Literally: At the eye

Example: “Le patron du bar nous a offert l'apéritif à l'œil.” — The bar owner gave us our drinks on the house.

English equivalent: On the house

Origin: In the first half of the 19th century this mostly meant on credit, not free. Gaston Esnault's reading is the simple one: a merchant extending credit to someone he does not really know is judging by appearance — by eye. From there, debtors being what they are, credit slid easily into free. Claude Duneton points out that the two senses ran side by side for a long time, citing Delvau in 1867, and traces the free sense to appearance as well: the shopkeeper who gives a small thing away to a pretty young customer, a habit attested in the 17th century and surely older. For the credit sense he goes further, though without texts to prove it, to a practice Furetière records — merchants tallied debts as knife-notches on a wooden stick, and the poor took bread à la coche, on the notch, until they could pay. A row of V-shaped nicks looks a good deal like a row of eyes. As literacy spread the stick gave way to a slate, which is where avoir une ardoise chez quelqu'un comes from. Two dead phrases preserve the credit sense: ouvrir un œil à quelqu'un, to extend credit, and crever l'œil à quelqu'un, to refuse it.