To worsen means to become worse or to make something worse than it was before. When a scraped knee worsens, it might get more infected or painful instead of healing. When a rainy day worsens, the drizzle might turn into a downpour with thunder and lightning.

You can worsen something yourself through your actions. Complaining loudly about a teacher's unfair rule might worsen the situation by making the teacher defensive instead of open to discussion. Scratching a mosquito bite worsens the itching. Waiting too long to study for a test worsens your chances of doing well.

Things can also worsen on their own. A patient's illness might worsen despite treatment. A friendship might worsen after a series of misunderstandings. Weather conditions can worsen quickly, turning a pleasant hike into a dangerous situation.