To let (someone) down

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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