Etymology
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impregnable (adj.)

early 15c., imprenable "impossible to capture," from Old French imprenable "invulnerable," from assimilated form of in- "not, opposite of" (see in- (1)) + Old French prenable "assailable, vulnerable" (see pregnable). With restored -g- from 16c. Related: Impregnably.

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Definitions of impregnable

impregnable (adj.)
immune to attack; incapable of being tampered with;
an impregnable fortress
Synonyms: inviolable / secure / strong / unassailable / unattackable
impregnable (adj.)
capable of conceiving;
Synonyms: conceptive
impregnable (adj.)
incapable of being overcome, challenged or refuted;
an impregnable argument
impregnable self-confidence
Synonyms: inexpugnable
From wordnet.princeton.edu