I think this is how you split it:

Pneumono-ultra-microscopic-silico-volcano-coni-osis

• Pneumono = lungs
• Ultra = very
• Microscopic = very small
• Silico = stone (silica)
• Volcano = …
• Coni = dust
• osis = medical condition

Please correct me if I’m wrong, I’m hearing this term for the first time so I might very well be off. Though I will say I am German and we do love our compound words, so I’m fairly confident this is at least getting the gist of things

So to conclude I think this just means “medical condition in the lungs caused by very fine volcanic dust”

How the -ism suffix would change the meaning, I’m not sure. I don’t think it would mean denial of said medical condition, though.