"For the love of Pete" and "For Pete's sake" - where do these sayings come from?

"For the love of Pete" and "For Pete's sake" are euphemisms for the phrases "For the love of God/Christ" or "For God's/Christ's sake" and hail from a time when those phrases were considered blasphemous. Nowadays phrases like "For the love of God" are commonly used, but the euphemisms are still used.

http://www.pride-unlimited.com/probono/idioms1.html

Why Pete? Most likely it is a reference to the Catholic Saint Peter.

'for God's sake'. Also 'for goodness' or 'heaven's' or 'Pete's' or 'pity's sake', are exclamations 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. The variants are euphemisms for God. (c. 1300)

'for the love of Pete' or 'Mike' or 'God', are exclamations of surprise, exasperation, or some similar feeling, as in For the love of Pete, give me the money! James Joyce used this idiom in Ulysses (1922): 'For the love of Mike listen to him.' Pete and Mike are euphemisms for God. (Early 1900s)

The American Heritage Dictionary of Idioms by Christine Ammer 1997 by The Christine Ammer 1992 Trust