FYI the phrase is “sacred cow”, not holy cow holy cow as a phrase is an exclamation comparable to “wow!” I can see where that gets confusing, though! I am aware your first language is not English, and let me assure you, outside of some idioms, your English is better than many native speakers!
No need to apologize at all. English's dual origins in Romance and Germanic languages makes it remarkably inconsistent. I am not surprised they would mean the same thing at all. Sacred and holy, when they are not next to the word "cow" mean the same thing in English as well.
holy cow as a phrase is an exclamation comparable to “wow!” I can see where that gets confusing, though! I am aware your first language is not English, and let me assure you, outside of some idioms, your English is better than many native speakers!
In fact both words translate to the same in Italian, I was retro-translating.