"Holy mackerel!” is an expression of surprise that has been cited in print since at least 1867. It has been suggested that the original expression was “Holy Mary,” with Christians replacing “Mary” with “mackerel.” It’s also been proposed that Protestants used this to mock Catholics, who eat fish on Fridays.
Similar food expressions (also a rhyme for “holy") include “Holy guacamole!” and “Holy cannoli!” “Holy jalapeño!” has been cited in print since at least 1977.
Mackerel is a common name applied to a number of different species of pelagic fish, mostly, but not exclusively, from the family Scombridae. They are found in both temperate and tropical seas, mostly living along the coast or offshore in the oceanic environment.
Mackerel typically have vertical stripes on their backs and deeply forked tails.
Wikipedia: Mackerel (food)
Mackerel is an important food fish that is consumed worldwide. As an oily fish, it is a rich source of omega-3 fatty acids. The flesh of mackerel spoils quickly, especially in the tropics, and can cause scombroid food poisoning. Accordingly, it should be eaten on the day of capture, unless properly refrigerated or cured.
Wiktionary: holy mackerel
Recorded from 1803 with uncertain origin, but possibly a euphemism for Holy Mary, with Mackerel being a nickname for Catholics because they ate the fish on Fridays. Another suggested explanation is the practice of selling mackerel on Sundays in the seventeenth century (because its quality deteriorates rapidly), so it was known as a holy fish.
(idiomatic, humorous or euphemistic) An expression of surprise.
Random House Historical Dictionary of American Slang: H-O
By Jonathan E. Lighter
New York, NY: Random House
holy mackerel! (used to express surprise, annoyance, or the like).
1885 in S. Crane Complete Stories 51: Holy Mackerel! I have gone and done it!
1894 Crane N.Y.C. Sketches 55: “Hully mack’rel,” said Billie.
1902 Hobart Woods 78: Suffering mackerel!
1919 Hinman Ranging in France 39: Holy mackerel, Buddy...we damned near got into Germany before we knew it.
1 May 1867, Herald Supplement (Boston, MA), “Our New York Letter,” pg. 2, col. 2:
“Holy mackerel!” replied the Jerseymen, “if you are looking for rabbits I am just the man to show you where you can find cart loads of them.”
17 August 1873, Sunday Mercury (New York, NY), “Sunday Table-Talk,” pg. 3, col. 3:
‘Twelve o’clock and all’s well,” died from the lips of the remotest guard, and I had risen to relieve my cramped limbs, when, holy mackerel! at the distance, probably, of two hundred yards, appeared, with appalling distinctness in the murky darkness, a ghostly white figure.
June 1876, Neighbor’s Home Mail, “He Was a Sailor Man,” pg. 67, col. 2:
“Holy mackerel!” says he.
28 July 1880, New Ulm (MN) Review, pg 2, col. 3:
Brown’s Valley Reporter: Last Sunday night was a night of terror to the human race in the Valley. The heat was terrible—not a breath of air stirred, and the mosquitoes, holy mackerel!
23 November 1881, The Herald and Torch-Light (Hagerstown, MD), pg. 1, col. 4:
“Holy mackerel! Gee Moses!”
(A poem titled “Love and Pain”—ed.)
Wild Woods Life, or, A trip to Parmachenee
By Charles Alden John Farrar
Boston, MA: Lee & Shepard, Publishers
“Holy mackerel!” exclaimed Maynard, — “two moose!”
OCLC WorldCat record
Holy mackeral ; Baby, don’t you want a man like me
Author: Little Richard
Publisher: [S.l.] : Modern Records, [between 1966 and 1967]
Edition/Format: Music : 45 rpm : Multiple forms : Popular music : Rock music : English
Now You Know Almost Everything:
The Book of Answers (Vol. III)
By Doug Lennox
Toronto, ON: The Dundurn Group
Why shouldn’t you say, “holy mackerel,” “holy smokes,” or “holy cow”?
As innocent as it seems today, “holy mackerel” began as a blasphemous Protestant oath against the Friday fish-eating habit of Catholics.