The phrase "For Pete's sake" is often referred to as a variety of euphemism which is used as an expression of annoyance called a 'minced oath.' A minced oath is a phrase that is substituted, sometimes at the last minute, for a swear word or blasphemous phrase. The minced oath usually sounds similar to the original swear word or expression. Examples abound with such ditties as oh ‘my goodness,’ gosh darn it,’ ‘shucks,’ ‘shoot,’ ‘fudge, ‘holy cow’ and ‘Mother Fletcher.’ For Pete's sake is probably a minced oath used in lieu of the more virulent forms "for God's sake," or "for Christ’s sake," or the milder "for Saint Peter’s sake."
Facts on File Encyclopedia of Word and Phrase Origins
The ‘Pete’ here is a euphemism for St. Peter, an expression of annoyance (“For Pete’s sake, stop fooling around!) probably dating back to at least the late 19th-century America.
Michael Quinion of World Wide Words
FOR PETE'S SAKE
Q. I was wondering if you can shed any light on "for Pete's sake". A very interesting explanation was given on a TV show, which attributed it to Michelangelo requesting funds for St Peter's. [Wayne R Zeides, USA]
A. Alas, that's a classic example of folk etymology. Not only has the speaker invented a story to explain a puzzling expression but he hasn't any feeling for history, or indeed geography. There is, for example, the annoying small fact that Michelangelo spoke a dialect of Italian and that we have to account not only for the phrase getting from Italy to (presumably) England, but for its swapping languages at least once along the way. There's also the problem that the expression isn't recorded before the 1920s, so where has it been in all the centuries in between? About the only thing he got right was that Michelangelo did work on St Peter's.
Pardon my irritation! It's easy to show such stories are nonsense, albeit usually well-meaning attempts to explain the inexplicable, but it's often hard to put anything sensible in their place. After all, these tall tales very often grow up because conventional scholarship has failed to find out the facts.
In this case, we're not totally without ideas. The clue is that another version of the exclamation is "for the love of Pete", which seems to be slightly older (it's recorded in print from 1918). In turn that reminds us of "for the love of Mike", which is older still, from the 1880s. This last expression seems to have been a euphemistic cry to replace "for the love of God", which is known from the early eighteenth century as an irritated exclamation. It looks very much as though at some point around 1918, for no clearly discernable reason, Pete joined Mike as the person to invoke when you were impatient, annoyed, frustrated or disappointed in someone or something, both men being stand-ins for the God that it would be blasphemous to mention.
Ken G – March 10, 2003
Reply from Ken Greenwald (Fort Collins, CO - U.S.A.)