Let down Definition 1 of 3Pro Tip 1/3Emotional Impact Using 'let down' indicates an emotional response to disappointment. After all the hype, their performance was a let down. Pro Tip 2/3Unmet Expectations It implies something or someone has not lived up to what was promised or expected. He didn't keep his promise, and I felt let down. Pro Tip 3/3Trust Issues 'Let down' can suggest a loss of trust in someone. By not backing me up, you've really let me down.