What does "inspissated gloom" mean?

Is "inspissated" used to simply emphasize the unhappiness/darkness? Is it used in literary contexts only? All the examples I have seen are either Biblical or literary.

Merriam-Webster

thickened in consistency; broadly : made or having become thick, heavy, or intense

Solution 1:

It is not a common word in any register. In Goodbye to All That Robert Graves tells a story about T.E.Lawrence which illustrates this point:

Professor Edgeworth, of All Souls’, avoided conversational English, persistently using words and phrases that one expects to meet only in books. One evening, Lawrence returned from a visit to London, and Edgeworth met him at the gate. 'Was it very caliginous in the Metropolis?'
  'Somewhat caliginous, but not altogether inspissated', Lawrence replied gravely.

I am pretty widely read, and I have never encountered the word outside this context.

Solution 2:

Inspissated is not only literary, it's actually a medical term as a dictionary tells you MW.

Medical Definition of inspissated : thick or thickened in consistency "blocked with inspissated bile" "the inspissated juices of an aloe"

However, connected with gloom it seems to be very rare and as you already discovered mainly in description of literary characters and literary scenarios.