The Eyeball Moose said:
"For the love of Pete" and "For Pete's sake".
Is Pete supposed to represent God? Did someone appoint Pete as the new God? That would make sense. If not, I don't know a Pete. Is Pete your friend? Why do you like him so much? I thought you loved ME. Are you seeing someone else? That's it, we're done. We're breaking up. Game over, *****.
'For God's sake!', 'For Christ's sake!' and by association 'For the Holy Ghost's sake!' just sounds dumb.
So, the third is just a prompt shortening of 'For St. Peter's sake!' Whether he is less offended, I don't know... nor do I care all that much, but that's where 'for Pete's sake!' comes from.