According to the Oxford English Dictionary Pneumonoultramicroscopicsilicovolcanoconiosis is "a factitious word alleged to mean 'a lung disease caused by the inhalation of very fine silica dust, causing inflammation in the lungs'". A condition meeting the word's definition is normally called just silicosis.
Wikipedia adds: "It occurs chiefly as an instance of a very long word. The 45-letter word was coined to serve as the longest English word and is the longest word ever to appear in an English language dictionary. It is listed in the current editions of several dictionaries".
Facetious or not its coining in 1935 by Everett M Smith appears well documented, and the word does indeed mean what the OED says.
Whatever you want to call the disease, you don't want it!