1580s, "of doubtful authenticity," from apocrypha + -al (1). Middle English had apocrive (late 14c.) in same sense. Related: Apocryphally.
Entries linking to apocryphal
apocrypha n.
late 14c., Apocrifa, "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 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).
The non-Biblical sense of "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.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).
*apo-
also *ap-, Proto-Indo-European root meaning "off, away."
It forms all or part of: ab-; abaft; ablaut; aft; after; apanthropy; aperitif; aperture; apo-; apocalypse; apocryphal; Apollyon; apology; apoplexy; apostle; apostrophe; apothecary; apotheosis; awk; awkward; ebb; eftsoons; of; off; offal; overt.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit apa "away from," Avestan apa "away from," Greek apo "from, away from; after; in descent from," Latin ab "away from, from," Gothic af, Old English of "away from."