Real Dictionary

let down
phrasal verb: If something lets you down, it is the reason you are not as successful as you could have been.
phrasal verb: If you let down something such as a tyre, you allow air to escape from it.
phrasal verb: If you let someone down, you disappoint them, by not doing something that you have said you will do or that they expected you to do.

let down in American English
to disappoint; fail
to lower

let down in British English
to lower
noun: a disappointment

let-down
variable noun: 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.