Definition: To not do something that other people are relying on.
1. (Of inflatable objects) To deflate something.
2. (Of clothes or curtains) To make longer by sewing the hem lower down.
E.g.1. You won’t let me down will you? I need you to come to this party with me.
E.g.2. He got tired of always being let down by her and left.
This phrasal verb can be separated.