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Webster 1913 Edition

adv.

In an impenetrable manner or state; imperviously.

“Impenetrably armed.”

Milton.

“Impenetrably dull.”

Pope.

adv.

With solidity that admits not of being penetrated.

With hardness that admits not of impression; as impenetrably dull.

impenetrably ‎(comparative more impenetrably, superlative most impenetrably)

impenetrably in The Century Dictionary, The Century Co., New York, 1911
impenetrably in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913