The deuterocanonical books of the Bible are those books in the Old Testament which are not found in the canon of modern, rabbinic Jews and Protestants. In Protestant circles, they are frequently referred to as “the apocrypha.”
“Apocrypha” means “hidden things,” and that’s a misnomer, because these books aren’t and never have been hidden. They were part of Christian Scriptures from the very beginning.
The term “deuterocanonicals” is also a misnomer, because its roots suggest these books belong to the “second canon,” and there is no second canon.
Alternately, one might parse it to mean that they were included in the canon secondarily–i.e., after other books–but this is also false. The canon lists of the early church councils in the fourth and fifth centuries–the first time the canon was dealt with by councils–include the deuterocanonical books alongside the protocanonical ones.
So, although it’s the term we’re stuck with, “deuterocanonicals” is itself problematic.
Today I did some research and was finally able to find out who coined the term: Sixtus of Siena.
You can read about him on Wikipedia here.
Based on information in the Oxford English Dictionary, it looks like the term was coined in or around 1566 in Sixtus’s work Bibliotheca sancta ex præcipuis Catholicæ Ecclesiæ auctoribus collecta (i.e., Sacred library collected from the precepts of the authorities of the Catholic Church).
The OED lists the following its first historical example of the term:
[1566 A. F. Sixtus Senensis Bibliotheca Sancta i. 10 Canonici secundi ordinis (qui olim Ecclesiastici uocabantur, & nunc à nobis Deuterocanonici dicuntur) illi sunt, de quibus, quia non statim sub ipsis Apostolorum temporibus, sed longè pòst ad notitiam totius Ecclesiæ peruenerunt, inter Catholicos fuit aliquando sententia anceps.]
While one must give the usual caveats about Wikipedia, it’s worth noting that it states:
Sixtus coined the term deuterocanonical to describe certain books of the Old Testament that had not been accepted as canonical but which appeared in the Septuagint, and the definer for the Roman Catholics of the terms protocanonical and the ancient term apocryphal.
I’d like to find a scholarly, non-Wikipedia source for these, but it does seem to jibe with the data from the OED.