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.

Definitions of sesquipedalian

n a very long word (a foot and a half long)

Type of:
polysyllabic word, polysyllable
a word of more than three syllables

adj given to the overuse of long words

sesquipedalian orators”
“this sesquipedalian way of saying one has no money”
making claim to or creating an appearance of (often undeserved) importance or distinction

adj (of words) long and ponderous; having many syllables

sesquipedalian technical terms”
primarily spatial sense; of relatively great or greater than average spatial extension or extension as specified

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