A let-down is a disappointment that you suffer, usually because something has not happened in the way in which you expected it to happen.
例句
The flat was really very nice, but compared with what we'd been used to, it was a terrible let-down...这套公寓真的很不错，但比起我们以往住的地方，那就差太远了。The sense of let-down today is all the greater because in the past doctors have been over-confident about these treatments.医生们过去对这些疗法过度自信，导致今天的挫败感尤其强烈。
英英词典释义
Noun
1. a feeling of dissatisfaction that results when your expectations are not realized;
"his hopes were so high he was doomed to disappointment"