[im.pen.e.tra.ble] adj [ME impenetrabel, fr. MF impenetrable, fr. L impenetrabilis, fr. in- + penetrabilis penetrable] (15c) 1 a: incapable of being penetrated or pierced b: inaccessible to knowledge, reason, or sympathy: impervious
2: incapable of being comprehended: inscrutable -- im.pen.e.tra.bly adv