So, I was saying congratulations to a friend the other day and noticed it was actually quite a long word. Four of my friends and I haven’t been able to find a longer word that the average person would know and use in ordinary conversation (i.e. the chemical listed on the back of the shampoo bottle doesn’t count).
Can anyone think of one?
I used discombobulated yesterday. Not longer, but more syllables anyway.
Depends on how you define common. “Constitutionality” is longer, and would be commonly found in newspaper articles.
What the stink does that mean?
Disproportionality?
Internationalization. While the average person might not use this in conversation, they would probably understand what it meant in context.
I’m sorry, I know this isn’t exactly common, but I absolutely had to add this one. My brother, my sisters and I used it all the time when we were kids to show how smart we were…
antidisestablishmentarianism
If anyone doubts the word…Link
I’ve actually managed to throw “pneumonoultramicroscopicsilicovolcanoconiosis” into a conversation but I wouldn’t call it common, exactly…
All of the halfway common words I can think of are topping out at 6-7 syllables. How about interdenominationally (9 syllables)? That’s found it’s way into some debates I’ve had on religion.
Good one, Valgard - my suggestion is Incomprehensibilities, equal in length and almost equal in syllables 
Misunderestimated? 
I’ve used antitransubstantiationalist in conversation, though I admit I was partly (only partly) motivated by the lure of sesquipedalia.
Well, in discussions of religious practice, I do use transubstantiation and its derivatives on a regular basis, perhaps an average of once a week – but you all knew I was odd, anyway!
Your admission shows admirable honorificabilitudinitatibus.
Floccinaucinihilipilificator!!! 
uncopyrightable
microminiaturisation
I’ve used hypoapobetalipoproteinemia in coversation before. Does that count?
It would be “smiles” because there’s a mile between the beggining and end of the word.
All of these high-brow answers make me embarrassed to mention…
supercalifragilisticespialidocious
It’s “smiles.”
Ask me why. Go ahead.