Jeter de l'huile sur le feu

Literally: To throw oil on the fire

Example: “Ils se disputaient déjà, et ta remarque a jeté de l'huile sur le feu.” — They were already arguing, and your remark added fuel to the fire.

English equivalent: To add fuel to the fire

Origin: Oil thrown on a fire does not put it out but makes it flare up, so the gesture makes anger or a quarrel worse instead of calming it. Wiktionnaire dates the phrase to the 17th century, notes that Madame de Sévigné used it, and finds it in old dictionaries including Furetière's (1690); the grammarian Féraud defines it in 1787 as irritating people who are already embittered. English has the same image.