A word made as what is straight up a giant middle finger to the 4 people in the world who actually have this condition.
This is how the word was probably invented.
Guy 1: Woah, slow down. Can you dumb down your explanation a bit, I don't understand, your words are too long, they sound like gibberish.
Guy 2: What's the problem? Are you afraid of long words?
Guy 1: Uhhh... sure? If that'll make you stop.
Guy 2: Oh, so you have a phobia of long words... long word phobia? Ah yes. You have Hippopotomonstrosesquippedaliophobia.
Guy 1: God fucking dammit.
1:Hey, Doc.
2:What ails you, son?
1:I fear long words.
2:Oh, you have HIPPOPOTOMONSTROSESQUIPEDALIOPHOBIA, but there is a simple fix, we ADMINISTER the VACCINATION into your ATRIOVENTRICULAR VALVE, where it-
1:AAAAAAAAAAAAAAAARRRRRRRRGGGGGGGHH!!!!!!!!!!!
Hippopotomonstrosesquippedaliophobia is a phobia which defines "the fear of long words." You probably wouldn't use this word in daily life.
Person 1: bro i have a fear of long words Person 2: so you have hippopotomonstrosesquippedaliophobia?
Person 1: *screaming in pain*
Person 2: i should not have said that.