Pneumono = concerning the lungs, like in pneumonia ultramicroscopic = smaller than microscopic silico = from silicon the element found in sand and rocks(and in computerchips!) volcano = from a volcanic eruption coniosis = lungdisease caused by inhalation of dust
Could it be that it is just the longest word in an English dictionary? And hence somewhat "constructed". You could simply speak of coniosis (already expensive enough) caused by very fine volcanic dust.