Lewis & Short

ignōrābĭlis, e, adj. [ignoro], unknown (rare but class.): separare, quod levius, facilius, non ignorabile, non fortuitum, non necessarium fuerit, * Cic. Inv. 2, 32, 99: libri litteris ignorabilibus praenotati, App. M. 11, p. 268: alterum illud ignorabilius obscuriusque est, Gell. 9, 12, 4 (Plaut. Ps. 2, 1, 18, false reading for ignobilis).
* Adv.: ignōrābĭlĭter, in an unknown manner: laminae ignorabiliter litteratae, i. e. covered with unknown characters, App. M. 3, p. 137, 6.