what does it all mean?
pies lesu domine dona eis requim.
FOR THE LOVE OF GOD WHAT IS HE SAYING?
The correct spelling is “Pie Jesu Domine, dona eis requiem.” It’s from the Latin Requiem Mass, and it means “Holy Jesus, Lord, give them rest”.
To see it in context with a full translation, try this site - this gives, specifically, the text used by Mozart in his setting of the Requiem, but he didn’t really leave a whole lot out.
This sounds like a clue from The Cryptic.
Monty Python!
Holly Gripes. Isn’t she the lead singer for the Dave Barry Band Name Conspiracy? 
When come back, bring pie jesu.
Pie Jesu? Mmmm…sacri-lcious.
Pie could be translated as ‘holy’ or, in the site you cite, ‘blessed.’ However, it’s usually translated as ‘sweet,’ as in “My sweet Lord! My sweet Jesus!”
However, a better sense akin to our modern idiom would be ‘dear.’ And so… “Dear Lord Jesus, let them rest.” (subjunctive)
Pax Christi.
“Holy gripes, Batman! It’s Latin?” – It’s English.
Yup, pius is a difficult word to capture with just one English equivalent… I was just too lazy to get into the whole thing 
Pius is, of course, the root of the word pious, and also carries with it the connotation of “dutiful”.
In the Catholic mass, though, pius is usually translated into English as “merciful”. At least in this particular passage.