The Word "Apocrypha"

The Greek word apo'kry'phos is used in its original sense in three New Testament verses, as referring to things "carefully concealed" (Mk 4:22; Lu 8:17; Col 2:3). in the first century A.D. when the term was applied to writings, it  referred to those that were to special to be read publicly, hence "concealed" from others.
      

The following explanation of the word "Apocrypha" is from Dr. Guthrie BD, MTH, PHD is a Protestant scholar and Lecturer in new Testament, at the London Bible College:

Their presence in the Vulgate is due to their inclusion, with exception of 2 Esdras, in the Greek translation of the OT, the Septuagint (LXX), which was the source of the Latin version of these books. It is commonly asserted that this fact shows that the Greek-speaking Jews of Alezandria gave them full canonicity, and that the primitive church, which took over the Greek Bible, did likewise. The books in question are largely of Palestinian origin and were mainly written in Hebrew or Aramaic; they were popular both in Palestine and in the Dispersion but seem to have been put on a different plane from the canonical Scriptures in all places. . . .

The Greek term [Apocrypha] means ‘hidden’ and was applied to books which were kept from the public eye and allowed to be read only by a privileged circle . Far from being an opprobrious term, therefore, it connotes the special value of the books so described. It seems to have been so applied to the works of the Jewish seers who were especially active between the 2nd century BC and the 1st century AD. These writings were issued under the name of ancient heroes and prophets of Israel and were kept hidden until those days; even so, they were not for the public but for those worthy to read them. 2 Esdras 14 relates how Ezra dictated to five scribes ninety-four books, twenty-four of which were the OT writing ( the Minor Prophets being considered as on book) and seventy being for ‘the wise among your people. For in them is the spring of Understanding, the fountain of wisdom, and the river of knowledge’ (14 : 46, 47). This shows that these books were valued ‘above’ the OT. . . . [ The New Bible Commentary: Revised, printed by Eerdmans, Copyright 1970, page 837]

The 1905 Jewish Encyclopedia Defines the word Apocrypha as:

The history of the earlier usage of the word is obscure. It is probable that the adjective abscondita "hidden away, kept secret," as applied to books, was first used of writings which were kept from the public by their possessors because they contained a mysterious or esoteric wisdom too profound or too sacred to be communicated to any but the initiated.

The Oxford Cambridge edition of the, Revised English Bible, in its preface to the "Apocrypha" tells us:

"THE term `Apocrypha', a Greek word meaning `hidden (things)', was early used in different senses. It was applied to writings which were regarded as so important and precious that they must be hidden from the general public and preserved for initiates, the inner circle of believers."