If something is a let-down, it is a disappointment. The image here is that you have built up your hopes. You are looking forward to something, and when it does not occur, you feel as if you have been...
Let your Hair Down: to behave freely and naturally.
Origin - This expression began in the 1800s when many women wore their long hair pinned up in public and only let it down in private, especially...
"Keep your hair on" could refer to the Old West, when Indians might scalp you if you were not watchful. I've never heard it said as "keep your hair on" though. I suppose the idiom 'keep your hair on'...
If you have something down pat, you understand it totally. This phrase can be used when you have learned a new skill, or memorized something. You say "I have that down pat," when you are confident...