The definition of impregnable is something that cannot be destroyed or entered by force.(adjective)
A fort that has been very well secured and that cannot be broken into is an example of something that would be described as impregnable.
See impregnable in Webster's New World College Dictionary
Origin: ME imprenable < OFr: see in- & pregnable
Origin: impregnate + -able
See impregnable in American Heritage Dictionary 4
Origin: Middle English imprenable
Origin: , from Old French
Origin: : in-, not (from Latin in-; see in-1)
Origin: + prenable, pregnauble, pregnable; see pregnable.
Origin: + -able.
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