The first time I came across the usage of this phrase was in the movie Hellraiser, and I had no idea this was a common saying. Clearly though, there must be a double meaning there in the movie that I couldn’t fully grasp without knowing the more colloquial meaning.
The description on Wikipedia is unfortunately not enough for me, I would like to see examples. And it’s very hard to find those because Google gives me mostly links to religious websites.
No, oath is just a tad archaic, but it’s a standard usage, not slang or dialect.
Iirc, that usage stems from the same as “swear” does, where the use of a “curse” word is an expression of emphasis or conviction, like saying “I swear to god, I’m going to kick this chicken into orbit if it pecks my foot again.”
Mind you, I haven’t gone looking for any rigorous history of that because it’s bloody difficult to search for on the internet. I’m basing that off of decades old time in a college with instructors willing to chat about their subject matter casually. It could have been the pet theory of the professor in question rather than an established fact.