To let down means to disappoint someone who was counting on you. When you let someone down, you fail to do what they expected or needed, breaking their trust or hope.

If you promise your younger sibling you'll help them build a treehouse on Saturday but then spend the whole day playing video games instead, you've let them down. If a friend tells everyone your secret after promising to keep it private, they've let you down.

Being let down hurts more than ordinary disappointment because it involves broken expectations. You didn't just lose something you wanted. Someone you believed in failed to deliver. A team lets down its fans when it plays without effort. A student lets down a teacher who spent extra time helping them prepare, then doesn't even show up for the test.

The phrase works as a noun too: experiencing a letdown means feeling that disappointment wash over you. After weeks of excitement about a camping trip, discovering it's been canceled feels like a real letdown.

Everyone lets someone down occasionally. We're human and make mistakes. People can recognize when they've let someone down, apologize, and do better next time.