1. disappoint someone, fail to support or help someone
let your family/friends/community down
As a phrasal verb to 'let down' means to descend before making a landing in an aircraft. To 'let down' something can also mean to alter the hem of clothing in order to make a garment such as a skirt or trousers longer. "I need to let down the hem of my son's school uniform each new term as he is growing so fast." You can also 'let down' the air in a tyre or something inflatable such as an air mattress or to move something into a lower position than it is currently in. "Let down the bucket from the window." To 'let someone down' means to disappoint them or fail to keep a promise. "I was let down when my date didn't show up at the cinema." If you 'let the side down' you disappoint a group or a team of people. "You really let the side down when you showed up late for the match on Saturday. We were all depending on you." If something is 'let down' by something else it means to make it less successful or effective. "The performance was let down by the bad sound at the venue." "The dinner was let down by the poor service in the restaurant.". In this way 'let down' can be used as a noun. "The sound was a let down." "The food was spectacular but the service was a let down." This is a noun and phrasal verb with a number of uses in every-day speech.