Cumulogranite

Imaginary type of cloud in aviation slang, describing terrain obscured by clouds that is posing a risk to the pilot.
The term is made up of „Cumulus“, a type of cloud and „Granite“, a type of igneous rock.

Most clouds you can fly through. Except Cumulogranite clouds. That ride, though short lived, is also the roughest.

Captain Careless had flown through many a cloud, but he only encountered a Cumulogranite right at the end of his life.