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impenetrable (adj.)

"impossible to penetrate," mid-15c., from Old French impenetrable (14c.) or directly from Latin impenetrabilis "that cannot be penetrated," from assimilated form of in- "not, opposite of" (see in- (1)) + penetrabilis "penetrable" (see penetrable). Related: Impenetrably; impenetrability.

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Definitions of impenetrable from WordNet

impenetrable (adj.)
not admitting of penetration or passage into or through;
an impenetrable fortress
impenetrable rain forests
impenetrable (adj.)
permitting little if any light to pass through because of denseness of matter;
impenetrable gloom
Synonyms: dense / heavy
impenetrable (adj.)
impossible to understand;
impenetrable jargon
From wordnet.princeton.edu