Holy mackerel is a lesser-known minced oath, similar to Geez Louise, however it is an euphemism of Holy Mary.
It used to refer to Catholics, who used to eat fish only on Fridays
Although it is not known where exactly “Holy mackerel” was conceived, its origins may likely stretch back to even the 18th century.
However, the first documented case of the minced oath dates back to the 1870’s, when it was included in the 1875 issue of Ballou’s Monthly Magazine’s 61st volume.
Following this, more and more books and magazines featured the expression at the end of the 19th century.
“Holy mackerel” slowly spread, gaining much prominence during the 1920’s, however its popularity started to die down following that.
“Holy mackerel” was first defined on Urban Dictionary on January 3rd, 2008, with several other entries to follow.
The expression was brought back to prominence in 2010, when the community-created melee weapon “Holy Mackerel” was introduced to Team Fortress 2 in the Mann-Conomy update, released on September 30th that year.
Since then, the expression has experienced a renewed popularity, being included in random exclamations, as well as literary works and even music.