احسن الله إليكم What is the ruling on saying “holy _____!” For something that is not holy?

This is forbidden without a doubt and it is also imitating the speech of the kuffar. The Prophet ﷺ said, whoever imitates a people is one of them (reported by Abu Dawud; graded hasan sahih by al-Albani rahimahullah). Therefore this is a dangerous matter that we should be very careful of and stay away from.