A feeling of disappointment, dissatisfaction, or frustration resulting from an unfulfilled expectation or a failed endeavor. From let down (to lower, to disappoint). The term 'letdown' has been used in English since the late 19th century to describe a feeling of disappointment, dissatisfaction, or frustration resulting from unfulfilled expectations or failed endeavors. It is derived from the phrasal verb 'let down', which means 'to fail to support or help someone when they are relying on you'.