let someone down

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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.
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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. [ ; mid-1700s ]
Also see: let down