Where did the phrase 'for pete's sake' come from?

I think that it is a euphemism for 'for God's sake'. One also hears 'for goodness' or 'heaven's' or 'pity's sake'. The euphemisms came about to avoid blasphemy. It's an exclamation showing surprise, impatience, anger, or some other emotion, depending on the context. For example, "For God's sake, I didn't expect to see you here", or "Hurry up, for goodness sake", or "For heaven's sake, how can you say such a mean thing?" or "For pity's sake, finish your dinner". It seems likely that 'for pity's sake' and 'for Pete's sake' are related. Another, similar, expression is "For the love of Mike".

before twenty or so years pete was a very common name... and hence originated "for petes sake", just like the average joe dat is used nowadays