I first heard this a couple of months ago, and I assumed that the clerk was being patronizing to a rather rude Spanish lady by telling her to "Get some rest and quit being so grumpy" but just now I heard it again in another show. Does it just mean "Have a nice day" or something? Very strange if that is the case.
It does mean something like "Get some rest." but it's not at all rude and I would say that it could be used like we'd say "Take it easy."
It can also be used for the English equivalent of 'sleep tight' although I found reference to it being used in Argentina as being similar in meaning to 'have a good day' - I'm not sure if this is just regional?
This is just the opposite of rude.
Que descanses is a short version of Espero que descanses bien.
Que pases buenas noches ................ Espero que pases buenas noches
Que tengas buen día ........... Espero que tengas ..............
I normally use ¡que descanses! at the end of a hard working day, or when I'm saying goodbye to someone that I know has been having a rough , tiring time. It means: may you have a restful evening / afternoon / weekend. I hope you are able to rest and relax and be restored.
í Qué descanses !
I would say that it means;
**í Qué descanses ! = relax / take it easy / have a rest ! **
Interestingly enough it is using the subjunctive mood -rather than the imperative - which suggests to me that the person speaking is wishing that someone enjoys a rest and is not a command so would not be considered rude
I have found descanso in my Spanish Oxford Dictionary (3rd Edition)
which means rest
and í Qué descanso, estaba tan preocupado ! = What a relief, I was so worried !
I hope that this helps