Those books included in the Septuagint and Vulgate versions of the Old Testament, but not considered to be parts of the original canon. They are accepted as canonical by Catholics, but not by Protestants, and are not printed in Protestant Bibles in ordinary circulation. The word means hidden (Greek, apokrupto), “because they were wont to be read not openly. ... but, as it were, in secret and apart” (Bible, 1539, Preface to the Apocrypha). As the reason why these books are not received as canonical is because either their genuineness or their authenticity is doubtful, therefore the word “apocryphal” means not genuine or not authentic.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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