Question: What is Pnemonoultramicroscopicsillicovolcanoconiosis?

I suspect you already know ;P
Having said that, it is the English language’s longest word at 45 letters and is a specific type of silicosis.
The word is constructed very carefully to describe exactly what it means
Pneumo – pertaining to the pneumothorax (throat above the lungs)
Ultramicroscopic – very small
Silico- Volcano – Silicon dust, generally from volcanoes
Osis – a suffix meaning disease

Silicosis is a lung condition caused by the inhalation of fine silica dust that causes the lung to inflame and scar at the points the silica lands. Extensive scarring leads to decreased lung function and can develop into lung cancer as many cells are growing and dividing.
It looks a bit like mesothelioma which is caused by asbestos but instead of the problem occurring in the lungs a layer called the mesothelium is affected. The mesothelium is found lining the internal organs of the body but is most pronounced around the lungs which is why breathing asbestos dust can cause mesothelioma.