What is the origin of term holy mackerel?

The term "holy mackerel" is believed to have originated in the early 1900s as a minced oath, a euphemism for a stronger or more offensive expression. It is thought to be derived from the phrase "holy mackerel, what a catch!", which was commonly used by fishermen to express surprise or wonder. Over time, the phrase was shortened to "holy mackerel" and became used more generally as an exclamation of surprise, frustration, or annoyance.