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.”

For more information, please visit www.languagesystems.com