My last post had me thinking about the expression “holy mackerel,” and I wondered how it originated. Why are mackerel any holier than any other fish, or any other animal for that matter? Fish in general seem to show up quite a bit in Christianity, as with the Jesus fish and the fact that some of Jesus’ disciples were fishermen. “Holy mackerel” in particular, however, might relate to the Catholic tradition of eating fish on Fridays. According to this answer, a derogatory term for Catholics in the nineteenth century was “mackerel-snappers.” Mackerel in particular might just be because it’s an amusing-sounding name, but it’s also been proposed that it could be a reference to either Mary or Michael the Archangel.
Speaking of fishy oaths, another one I’ve come across occasionally is “odds fish,” which was apparently uttered by the Scarlet Mackerel…uh, Pimpernel. I’ve also come across it in the Oz books, in which it’s one of Sir Hokus’ knightly expressions. “Odds,” in this case, is a minced form of “God’s,” and it makes as much sense for God to have fish as it does bodkins or goblins. An “odd fish” can also be a term for an unusual person, sort of like “odd bird” or “queer duck” (although I suspect that latter isn’t used in its original context so much anymore), but I don’t know if there’s a connection.