- impenetrable (adj.)
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[impenetrable 词源字典] - mid-15c., from Middle French impenetrable, from Latin impenetrabilis "that cannot be penetrated," from assimilated form of in- "not, opposite of" (see in- (1)) + penetrabilis "penetrable" (see penetrate). Related: Impenetrably; impenetrability.[impenetrable etymology, impenetrable origin, 英语词源]