I came across this phrase: Holy mac and cheese
What does it mean actually? Is it a slang term or an idiom?
I came across this phrase: Holy mac and cheese
What does it mean actually? Is it a slang term or an idiom?
This is what’s known as a minced oath. It’s a euphemism that is formed by someone wishing to pull back from cursing, either out of genuine deference to propriety or (more often) for humorous effect.
I haven’t encountered this particular one before, so it definitely hasn’t reached the level of idiom.
As it is not at all formal or reverent, it could certainly be called slang.
I believe Adam presented the correct hypothesis as a comment, that this particular phrase was formed because it sounds like “holy mackerel” which may itself be a euphemism for “holy Mary” or may be a reference to the Catholic practice of consuming fish on Fridays.
As FumbleFingers notes in a comment below, the word “cheese” is commonly used in euphemisms like this in reference to the similar-sounding name of Jesus.
Because it has come up in that comment and another, I will also mention that mac and cheese is a thing unto itself and commonly referred to as such in AmE. I do not believe this is a reference to a Big Mac, which, as Jasper points out, “includes cheese by default.”
The two most common forms of "swearing" in most languages are references to bodily functions and blasphemy. Common epithets include "Holy Mother of God" (or just "Holy Mother" or "God"), "Jesus Christ", "Holy Mary and Jesus", and so on. Phrases like "Holy Mackerel", "Gosh", "Jeepers", "Criminy", and "Holy Mac and Cheese" are euphemisms for these blasphemies.
Such interjections or invocations as Holy Mother of God used as an expression of surprise or fear have a lot of variations and new ones are formed continually.
Examples: Holy Moses (variant: Owly mouses), Holy mackerel, Holy cow, Holy shit etc.