It’s time to let the cat out of the bag. Although there are some fascinating idiom origins, one that people might also be curious about is, “for Pete’s sake!” And, for Pete’s sake, you should.
The phrase essentially uses “Pete” as a mild substitute for God or Christ in an expression of annoyance or frustration. It’s similar to the less common, “for the love of Mike.” In either case, the switch to the common name makes the phrase more socially acceptable and less offensive. Replacing the words in a euphemism to make a saying more appropriate is known as a “minced oath.”
People started saying “for Pete’s sake” as early as 1903, according to Oxford English Dictionary citations. But there’s no confirmed reason why people use Pete instead of Tom, Jim, or any other name for that matter. One speculative theory in the Morris Dictionary is that someone replaced Jesus or God with another religious figure—St. Peter.