Impregnable

1. adjective

   immune to attack; incapable of being tampered with

   “an impregnable fortress”

   Similar: inviolable, secure, strong, unassailable, unattackable

2. adjective

   incapable of being overcome, challenged or refuted

   “an impregnable argument”

   Similar: inexpugnable

3. adjective

   capable of conceiving

   Similar: conceptive