It's a well known old saying, present in pretty much all languages and cultures I'm aware of, meaning exactly that one would be angry or/and upset, had one been alive today to witness whatever the saying relates to.
Only the proper way would be saying "rolls over in his grave", not spins, but, I guess, it's a 'postmodern' intensification.
In slavic languages it literally translates "turns over in his coffin", but basically the same thing.
Oh, I'm well aware of the saying and what it is supposed to mean.. My point is it has never made any sense.
Somebody made a joke a long time ago when they came up with the saying.
It just makes more sense to say that "Gene
is spinning in his grave" or that somebody "
is turning in their grave" about something.
To say somebody would be turning in his grave
if he were alive today...well duh...wouldn't you be if you were buried alive? No need for somebody to ruin your creation to be upset about being alive in a coffin buried underground.
Actually I believe the original saying probably is "he's turning in his grave" without any mention of said person being alive.
Somebody probably misquoted it and others have been saying it wrong ever since.
Like a Biff.