To learn the meaning of a word or phrase means knowing its many uses, especially the more common ones. Words like run, get, put, and fall have over 200 definitions in the Oxford dictionary! The word let is also a very common English word and when partnered with different words generate different meanings.
In fact, when combined with down, the verb form to let down has two meanings. (Only the first meaning will be discussed here. Stay tuned for my next entry for the second meaning.) The more obvious one is to bring something down like curtains like “Please let down the curtains.” This is not too difficult to understand as the curtains should then be lowered or move downwards by the listener. Notice that the object (curtains) is placed behind the verb (let down).
There is a famous German fairy tale about a girl named Rapunzel who was trapped in a tower in the middle of the woods but whose songs attracted a prince. He called out to her, “Rapunzel, Rapunzel, let down your hair.” And, when she did, he climbed it to ask her to marry him as her hair was long and strong like a rope. It is a Cinderella-like story where this line has become popular.
Stay tuned for the second and more abstract meaning of let down!