aigrir

Aigrir means to turn sour or bitter — used literally for milk or wine going off, but far more often figuratively in the reflexive s'aigrir, describing a person growing bitter and resentful over time, typically after repeated disappointment or failure.

• Le lait a aigri parce qu'il est resté trop longtemps hors du frigo. — The milk turned sour because it was left out of the fridge too long.
• Il s'est aigri après des années de déceptions professionnelles. — He grew bitter after years of professional disappointments.
• Ne laisse pas cet échec t'aigrir. — Don't let this failure make you bitter.

Learners often reach for devenir amer instead of the more idiomatic reflexive s'aigrir when describing someone growing bitter over time — s'aigrir is the natural verb French speakers use for this gradual emotional souring.