Se mettre la rate au court-bouillon

Literally: To put one's spleen in the poaching broth

Example: “Ne te mets pas la rate au court-bouillon pour un simple contrôle de routine.” — Don't work yourself into a state over a simple routine checkup.

English equivalent: To work oneself into a lather

Origin: Old medicine held that the body ran on four humors, and that the spleen (la rate) produced black bile, the humor that, in excess, made people gloomy, anxious and melancholic. French still carries traces of that belief: se faire de la bile ("to make bile") means to worry, and se dilater la rate ("to stretch one's spleen") means to laugh heartily. A court-bouillon is the seasoned liquid in which fish or meat is poached at a steady simmer. Putting your spleen in it pictures your organ of worry slowly stewing: you are cooking yourself in your own anxiety. Despite the old medical image, the phrase is modern colloquial French: the earliest example found is in Robert Gaillard's novel La castille d'or (1952), and it is often linked to the San-Antonio crime novel La rate au court-bouillon (1965), which may have helped spread it. A story tying it to sailors' slang circulates online, but no source supports it.