Wiktionary
rest his soul
interj. (context idiomatic English) (non-gloss definition: Used parenthetically to mark the referent as being deceased)
Usage examples of "rest his soul".
But for a journey when she was a small child, when her pa, rest his soul, took them off to Fairfield, for what she could no longer remember, she had never even been up there.
Yes, at this moment, when I see a fustanella in my yard once more, I suddenly remember 1866, and how in this very yard, in the chair in which you are sitting, Captain Liapes (God rest his soul!