Payer rubis sur l'ongle

Literally: To pay ruby on the nail

Example: “Le collectionneur a payé la voiture rubis sur l'ongle.” — The collector paid for the car cash on the nail.

English equivalent: Cash on the nail

Origin: The 17th century said faire rubis sur l'ongle, not payer. Philibert-Joseph Le Roux explains it in his Dictionnaire comique of 1718: drinking a toast to an absent friend, you kept a last drop in the glass, tipped it onto your thumbnail and licked it off, to show the regard you held them in. In a glass of red, that drop passes easily for a ruby. At the same period the phrase became a metaphor for paying to the last sou — pockets emptied as thoroughly as the glass. The liquid ruby was forgotten, the money sense survived, and payer pushed out faire.