What is the meaning of the word hippopotomonstrosesquipedalian?

It means 'pertaining to the quality of being a long Word'.

This is why it can be used in comparative and superlative form.

Example (Superlative): "That's the most hippopotomonstrosesquipedalian Word I've ever seen!"

Example (Comparative): "That Word is more hippopotomonstrosesquipedalian then the the real name of the protein Titin!!"