To rest in a state of blessed, undisturbed peace, often implying spiritual comfort or repose after death; originally a line from the Christmas carol 'Silent Night.'
2.
General Englishinformalcolloquialironic
A short, often jocular or sarcastic refrain used to indicate that someone or something should be left undisturbed, allowed to remain at peace, or dismissed from further concern.
Examples:
EN: * sleep in heavenly peace...
ES: Brilla la estrella de paz
EN: ♪ sleep in heavenly peace ♪
ES: ♪ Duerme en paz celestial ♪
EN: - Mother... ♪ Holy infant, so tender and mild ♪ - Son of a bitch! ♪ sleep in heavenly peace ♪