‘Apocrypha’ means ‘hidden’. Why were those books ever called the Apocrypha?

This text is from a transcript of a talk by David Gooding, entitled ‘Where Did the Old Testament Come From?’ (2006).

As you say, the word means 'hidden away' but it came to be used in a different sense, and I'm not quite sure myself what really the sense is. The immediate meaning of apocrypha normally means something 'crypha', 'hidden away', Apo-crypha. Sometimes it is used of the books of hidden wisdom, particularly among mystics, but that is not the meaning claimed for these particular books. So why they continued to use it is a mystery to me. I would have to refer you to the technical works of Bruce Metzger and F. F. Bruce on the Apocrypha and so on.

Yes, it's a bit of a mystery. And you notice that the modern Roman Catholic scholars, particularly, have abandoned the use of this term. They talk about the deuterocanonical books and then, if they use the Apocrypha, they use the many, many other books outside that list of so-called apocryphal books.