1.  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.
2.  let someone down easy. Convey bad or disappointing news in a considerate way, so as to spare the person's self-respect. For example, The teacher knew that Paul would have to repeat the course and that there was no way to let him down easy. [Colloquial; mid-1700s] Also see let down.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.