Holy Cross
Specifically, Holy Cross is used to refer to the True Cross on which Jesus was crucified.
In general, Holy Cross distinguishes the cross as religious and not simply a decoration. For anything 'holy', the religion does not have to be Christian - the Qur'an and Torah are holy books and we all enjoy our holydays.
The origin of the word 'holy' is obscure: it could have evolved from the Old English hālig, which means 'wholeness' (where the first 'e' is pronounced 'i'). It is derived from Proto-Germanic hailagaz, which had a similar meaning, referring to something that was sacred, blessed, or inviolable./p>
The word hailagaz can be traced to the Proto-Indo-European root kailo, which means "whole," "uninjured," or "healthy." This idea of being "whole" or "unblemished" evolved into the meaning of being "sacred".
Alternatively, 'holy' may be derived from Old High German hulis, meaning 'holly'. The holly tree was considered a sacred plant in both pre-Christian Celtic and Roman religions. Why did they considered the holly tree sacred?
A popular understanding is because the leaves have spikes which protect the plant from being eaten by animals. Yet lots have plants have spiky bits for protection; for example, Juniper, Hollyhock and Ruscus, and some or them have red berries; for example Berberis.
So we don't know for sure why holly was singled out specifically as sacred, or why the Romans connected holly to Saturn, the god of agriculture.
Whatever the word's origin, sadly today, "holy [expletive]" is used more frequently in casual, colloquial speech, than "holy scriptures" or any other thing recognized as sacred, godly, saintly, and spiritually pure.