where does the expression "to let down" come from?
The simple (and as I think) original meaning is "to lower", is it not?

Does it come from mining, where someone, who is let down into the dark mine shaft, feels  depressed/miserable, in short: let-down??

Does it not rank as one common expression from the endless annals of bourgeois courtship?  When a truly fetching woman leaves her downtrodden husband or fiance in the veritable lurch, that is without any customary means to social recourse: "She really let him down by taking him for everything he got, and leaving the poor bloke without any mutual friends to console him!"

A sic heretic would say that it is an abbreviation of "left to drown".

Well, not to expect any redemptive contrition from your once sworn EX if you know what I mean!  You might as well bring along one of those big, dry, fluffy HIS towels from your new, slightly downsized washroom—cheers, pal!

Also, let-down.

Origin: 1760–70; n. use of v. phrase let down

Idiom(s):
let (one’s) hair down - To drop one’s reserve or inhibitions.

http://dictionary.reference.com/search?q=let+down&r=66

1.  Cause to descend, lower, as in They let down the sails. [Mid-1100s]
2.  Also, let up. Slacken, abate, as in Sales are letting down in this quarter, or They didn’t let up in their efforts until the end. The first term dates from the mid-1800s, the variant from the late 1700s.
3.  See let someone down. Also see let one’s hair down.

http://www.answers.com/let down

Idioms: let someone down

1.  Fail to support someone; also, disappoint someone. For example, I was counting on John to come, but he let me down, or The team didn’t want to let down the coach. [Late 1400s]
A British phrase with the same meaning is let the side down, alluding to some kind of competition (sports, politics) and dating from the mid-1900s. It is occasionally used in America.
2.  let someone down easy. Convey bad or disappointing news in a considerate way, so as to spare the person’s self-respect. For example, The teacher knew that Paul would
have to repeat the course and that there was no way to let him down easy. [Colloquial; mid-1700s]

http://www.answers.com/topic/let-someone-down

Idioms: let one’s hair down

Also, let down one’s hair. Drop one’s reserve or inhibitions, behave casually or informally, as in Whenever the two sisters get together, they let their hair down and
discuss all their problems. This expression alludes to the practice of women taking down their pinned-up long hair only in the privacy of the bedroom. [c. 1900]

http://www.answers.com/topic/let-one-s-hair-down

allow hair to hang down

http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861696275

Grown women used to wear their hair up in a bun or whatever. Hair was only “let down” at night or when they were going to wash it.

The point of the idiom is that it was improper for a grown woman to appear in public with her hair loose, so she could only let her hair down in an informal situation when only family or intimate friends were present. A young man might ask his fiancee if he could see her with her hair down - that was quite a sexy request, which he couldn’t decently make before they were engaged.

http://www.phrases.org.uk/bulletin_board/59/messages/186.html

Alas, you better not expect any forthcoming redemption either—professor!