To embitter means to make someone feel angry, resentful, or cynical, especially after a disappointing or unfair experience. When something embitters you, it leaves you with a bitter, sour feeling that can last a long time.

Imagine a student who works incredibly hard on a science fair project, follows all the rules carefully, but loses to someone who clearly had their parents do most of the work. That unfair outcome might embitter the honest student, making them feel that hard work doesn't matter. Or think of a coach who gets fired unfairly after years of dedication: the experience could embitter them toward the sport they once loved.

When you're embittered, it's like bitterness has soaked into your outlook. An embittered person has usually experienced real disappointment or injustice, and those bad experiences have changed how they see the world.