Use the adjective sesquipedalian to describe a word that's very long and multisyllabic. For example the word sesquipedalian is in fact sesquipedalian.
Sesquipedalian can also be used to describe someone or something that overuses big words, like a philosophy professor or a chemistry textbook. If someone gives a sesquipedalian speech, people often assume it was smart, even if they don’t really know what it was about because they can’t understand the words. Each of those long words is referred to as a sesquipedalia. Antidisestablishmentarianism is a sesquipedalia: in fact it’s the longest non-coined and nontechnical word in the English language.