I don't have access right now to some of the sites I usually use for information about expressions, so I have to ask you all: Is there any difference between the expressions "turn someone down" and "let someone down"? And if yes, what is it? I know that the second one means "to disappoint someone" but I'm not sure what the first one means.

To 'turn someone down' usually means to deny or decline a request from them, where as 'letting someone down' generally has the connotation that you mentioned, to disappoint someone.

Like if you asked me to go on a date, I say no, you could say "He turned me down." If I said yes, then failed to show up for the date, you might say "He really let me down."

Yeah FatAndFaithless is right. Also turning someone down usually is in regards to things like dates or romantic situations.

To turn someone down is to reject an offer, "No, I won't go to the prom with you."

To let someone down is to disappoint them, "Sorry, I never finished installing the escape ladders from the mine."

I see. Thank you for the answers. I didn't know that the first one is related to requests.

You can let yourself down, but you can't turn yourself down

(August 16, 2016 at 11:13 am)Alasdair Ham Wrote: You can let yourself down, but you can't turn yourself down

I could turn myself down, if I had a clone of myself. Not that I would, though.

One typically turns down offers, for example

I chose to turn down the request in the OP. Sorry to let you down.

(August 16, 2016 at 11:13 am)Alasdair Ham Wrote: You can let yourself down, but you can't turn yourself down

Of course you can.  How do you think I lost all that weight?