"The expression "son of a gun' is an euphemism for 'son of a b*tch'." I highly doubt this. I strongly suspect the two developed independently. I have no support for saying so, though, but, then, neither do you provide any support for your claim to the contrary.

FWIW see https://www.etymonline.com/word/son

@msh210 support for "The expression "son of a gun' is an euphemism for 'son of a b*tch'": Merriam-Webster (https://www.merriam-webster.com/dictionary/son%20of%20a%20gun) Cambridge dictionary (https://dictionary.cambridge.org/us/dictionary/english/son-of-a-gun) Collins dictionary (https://www.collinsdictionary.com/dictionary/english/son-of-a-gun, see AME section)

Well, then, @Moshi , there's the origin. Someone took "son of a bitch" and substituted a word he didn't like. What's the question, then?

@Moshi Thanks for the links, I've added them to the question.

@msh210 I just wanted to provide some context about the expression's meaning. But if it's not relevant to the question (which is "what's its origin?"), I can remove the first paragraph.

Not sure what you're asking, really. If it's known to be a euphemism, isn't that its etymology? Or do you mean as follows? "Son of a gun has an older sense, which I'm seeking the etymology of. Later, it was also used as a euphemism…."

@msh210 I've edited the question again, hope it's clear now.