to make someone feel unhappy because something they hoped for or expected did not happen or because someone or something was not as good as they expected （使）失望
I hate to disappoint you, but the cake’ s all gone. 我真不愿让你失望，不过蛋糕吃完了。
disappoint sb’ s hopes/expectations No one wants to disappoint people’ s hopes of peace. 没有人想让人们的和平希望破灭。