immune to attack; incapable of being tampered with; an impregnable fortress; fortifications that made the frontier inviolable; a secure telephone connection

[ Syn: inviolable , secure , strong , unassailable , unattackable ]

capable of conceiving

[ Syn: conceptive ]

incapable of being overcome, challenged or refuted; an impregnable argument; impregnable self-confidence

[ Syn: inexpugnable ]