Holy mackerel

An exclamation of surprise, alarm, or amazement. Mild, comic, and somewhat old-fashioned Regional use: United States, later wider English.

A humorous American minced oath: holy is paired with an absurd, harmless noun to avoid a potentially blasphemous exclamation. Holy mackerel is securely printed in a New York newspaper in 1853. The fish itself has no demonstrated connection to Lenten meals or older slang; those explanations impose significance where the historical evidence suggests playful randomness.