Fail to support someone; also, disappoint someone. For example, I was counting on John to come, but he let me down, or The team didn't want to let down the coach. [Late 1400s] A British phrase with the same meaning is let the side down, alluding to some kind of competition (sports, politics) and dating from the mid-1900s. It is occasionally used in America.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.