To let (someone) down: to disappoint, to not complete an expectation
Jamie: Sorry I’m late, Mr. Tosca.
Mr. Tosca: You told me your presentation would ready two hours ago. Our new clients are very angry.
Jamie: Don’t worry, Mr. Tosca. I’ll explain everything to them.
Mr. Tosca: No, you won’t. They left 45 minutes ago! You really let me down.
“Let down” is also used as a countable noun to mean “disappointment.”
Jake: Did you like the movie?
Linda: No, it was a let down.
“Let down” is a common phrase in popular culture. Search for the Beatles’ song “Don’t Let Me Down.”
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