Knock-knees: An abnormal curve of the legs that causes the knees to touch or nearly touch while the feet are apart. The problem may arise in the bone structure, or it may develop gradually as a result of muscle abnormalities. Knock-knees can cause movement difficulties, muscle and bone strain, and pain due to overstress on the ankles. The condition can be treated via physical therapy, and in some cases it can be corrected with surgery. Also known as genu valgum.