To defeat of expectation or hope; to hinder from the attainment of that which was expected, hoped, or desired; to balk; as, a man is disappointed of his hopes or expectations, or his hopes, desires, intentions, expectations, or plans are disappointed; a bad season disappoints the farmer of his crops; a defeat disappoints an enemy of his spoil
fail to meet the hopes or expectations of; "Her boyfriend let her down when he did not propose marriage
fail to meet the hopes or expectations of; "Her boyfriend let her down when he did not propose marriage"
If things or people disappoint you, they are not as good as you had hoped, or do not do what you hoped they would do. She knew that she would disappoint him. = let down