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im·preg·na·ble 1

1. Impossible to capture or enter by force: an impregnable fortress.
2. Difficult or impossible to attack, challenge, or refute with success: an impregnable argument.

[Middle English imprenable, from Old French : in-, not (from Latin in-; see in-1) + prenable, pregnauble, pregnable; see pregnable.]

im·preg′na·bly adv.

im·preg·na·ble 2

Capable of being impregnated.

ThesaurusAntonymsRelated WordsSynonymsLegend:
Adv.1.impregnably - in an impregnable manner; "the sight of that bland, impregnably righteous face has been enough to make their blood run cold"
References in classic literature ?
Finally, though, as will soon be revealed, its contents partly comprise the most delicate oil; yet, you are now to be apprised of the nature of the substance which so impregnably invests all that apparent effeminacy.
cfm) President Herbert Hoover  lauded "the willingness of our people to accept this added burden in these times in order impregnably to establish the credit of the federal government.
He said Prime Minister was punished on making country's defence impregnably.
At their zenith they appear impregnably all-powerful.