Impenetrably

Far otherwise the inviolable Saints, In cubick phalanx firm, advanced entire, Invulnerable, impenetrably armed;

Source: wiktionary

And now, on Fancy’s easy wing convey’d, The King descended to th’ Elyzian shade. There, in a dusky vale where Lethe rolls, Old Bavius sits, to dip poetic souls, And blunt the sense, and fit it for a scull Of solid proof, impenetrably dull.

Source: wiktionary

She looked at Jane, to see how she bore it; but Jane was very composedly talking to Bingley. She looked at his two sisters, and saw them making signs of derision at each other, and at Darcy, who continued however impenetrably grave.

Source: wiktionary

And apart from the manifold ruffianism, shrouded impenetrably under the mixed hats and turbans of men whose past was untraceable to any European eye, it is well known that the navy (especially, in time of war, the commercial navy) of Christendom is the sure receptacle of all the murderers and ruffians whose crimes have given them a motive for withdrawing themselves for a season from the public eye.

Source: wiktionary