Lewis & Short

ĭn-excūsābĭlis, e, adj., that cannot be excused, inexcusable (poet. and post-class.): ne te retrahas, et inexcusabilis absis, Hor. Ep. 1, 18, 58: tempus, Ov. M. 7, 511: onera, that cannot be refused, Dig. 5, 1, 50: necessitas, Cod. Th. 11, 16, 7 al.
Comp., Ambros. de Vocat. Gent. 2, 4.