apocrypha (n.)

late 14c., Apocrifa, in reference to the apocryphal books of the Bible, from Late Latin apocrypha (scripta), from neuter plural of apocryphus "secret, not approved for public reading," from Greek apokryphos "hidden; obscure, hard to understand," thus "(books) of unknown authorship" (especially those included in the Septuagint and Vulgate but not originally written in Hebrew and not counted as genuine by the Jews), from apo "off, away" (see apo-) + kryptein "to hide" (see crypt).

Non-Biblical sense "writing of doubtful authorship or authenticity" is from 1735. Properly plural (the single would be Apocryphon or apocryphum), but commonly treated as a collective singular.