Lewis & Short

authentĭcus, a, um, adj., = αὐθεντικός, that comes from the author, authentic, original, genuine (in the jurists and Church fathers; syn.: verus, germanus): testamentum, the original will, Dig. 29, 3, 12: tabulae, the same, ib. 10, 2, 4.
Also subst.: authentĭcum, i, n., the original writing, the original, Dig. 22, 4, 2.