Lewis & Short

ŏpisthogrăphus, a, um, adj., = ὀπισθόγραφος, written on the back (as, of the paper, etc.), opisthographic (post-Aug.): commentarii, Plin. Ep. 3, 5, 17.
Subst.: ŏpisthogrăphum, i, n., that which has been written upon on the back: testari in opisthographo, Dig. 37, 11, 4.