Definition of "caliginous"

Hierapolis is ſeated over-againſt Laodicea, where are to be ſeen baths of hot waters, and the Plutonium. [...] The Plutonium is under the brow of the hill, the entrance into which is no wider than that a man can thruſt himſelf through; yet it is very deep within, of a quadrangular form, containing about the compaſs of half an acre, and is filled with ſuch a thick and caliginous air, that the ground cannot be ſeen.

1809, Edward Wells, “Of St. Paul’s Travels and Voyages into Phrygia, Galatia, Mysia, Troas, Macedonia, Achaia, &c. till His Fourth Return to Jerusalem, after His Conversion”, in An Historical Geography of the Old and New Testament: In Two Volumes, volume II, Oxford: At the Clarendon Press, section I (Of St. Paul’s Travels, from His Leaving Jerusalem, after the Council there Held, to His Departure out of the Asiatic Continent for Europe), page 258

By the time breakfast was announced, the land had faded into a thin caliginous streak; and, except passing a huge unwieldy Chinese junk, which lay at anchor, though her lateen sails were hoisted, nothing worthy of note occurred during the day.

1869 November, “The Land of the Malay: A Record of Travel in the Oriental Tropics”, in [Thomas] Mayne Reid, editor, Onward: A Magazine for the Young Manhood of America, New York, N.Y.: Onward Publishing Office, page 491

Inside the atmosphere was rank and caliginous: fumes rose from puddles, groans sifted through the shadows.

1981, T[homas] Coraghessan Boyle, “The Niger”, in Water Music, Boston, Mass.: Little, Brown; republished London: Granta Books, 1998, page 155

They say the darkest hour is just before the dawn, and caliginous thoughts often swirled through his murky, insomniac mind as he lay awake [...]

2010, Francis Wheen, Strange Days Indeed: The Golden Age of Paranoia, paperback edition, London: Fourth Estate, HarperCollins Publishers, page 21