mid-15c., from Middle French impenetrable (14c.) or directly from Latin impenetrabilis "that cannot be penetrated," from assimilated form of in- "not, opposite of" (see in- (1)) + [..]

unable to be pierced (penetrated) or understood.

not admitting of penetration or passage into or through; "an impenetrable fortress"; "impenetrable rain forests" dense: permitting little if any lig [..]

not penetrable The fortress is , so it cannot be taken.

(adj) not admitting of penetration or passage into or through(adj) permitting little if any light to pass through because of denseness of matter(adj) impossible to understand

Not penetrable. ''The fortress is impenetrable, so it cannot be taken.'' *(??????????????), (?????????????) * Spanish:la|impenetrabilis.

lang=en 1800s=1818
1843 * '''1818''' — . ''''. *: Sometimes I endeavoured to gain from Frankenstein the particulars of his creature's formation, but on this point he was impenetrable. * '''184 [..]