It's a humorous example of a long word and is not an actual medical term.
Let's break it down:
* Pneumono: Related to the lungs
* Ultramicroscopic: Extremely small, beyond the reach of a microscope
* Silico: Referencing silica, a type of mineral found in sand and dust
* Volcano: A mountain that erupts with molten rock and ash
* Coniosis: A lung disease caused by dust inhalation
The word was created as a joke to demonstrate how long and complicated words could be made, and it has become a popular example of a tongue-in-cheek word.