What does 'Rest in peace' mean. You only hear it from religions. Mostly Christian. On one hand, I'm supposed to get an afterlife, on the other I rest in peace. Which one? Is this another dogma contradiction? If afterlife invloves resting in peace, it doesn't sound like much fun. Unless you're a cat. A few thousand years resting could get kinda old. Eventually you'd want to do something. I suppose this is like concentrating on the void (in meditation) in order understand everything. A void is empty, rest = lack of activity. How does that lead to everything. Anyway, seems pretty paradoxical to me.
I believe they are referring to the other half of the self: The body. i.e. may your body rest in peace here, and may your spirit move on to heaven, something like that. Though I am not Christian or of any religion that believes in this prospect, so my explaination may not be accurate.
The way I understand it, is that we wish the spirit moves on further on its journey to its next life easily and comfortably, rather sticking around as a ghost or something.