Well, crap.
One of my buddies threw this one out in a game of Dictionary a few years ago, and was able to back it up with a reference in the OED. Two days later I came across the same word in a Heinlein novel.
DAMMIT! You beat me to it!
Yeah, I used to dazzle my elementary school mates with that little bit of useless trivia in my youth. Hey, I still think it’s a damn good word.
I looked through a list that is said to be the 1000 most common English words here, and there aren’t many long ones. The first 6-letter words are ‘number’ (76) and ‘people’ (80). There are a few longer words such as ‘probably’, ‘sentence’, ‘different’ in the 100-200 range. The whole list doesn’t seem to have anything longer than ‘information’.
One of the chemicals often found on the back of shampoo bottles (and it might be the one Cast has in mind) happens to be one I enjoy saying: methylchloroisothiazolinone. It has antibacterial and antifungal properties and is used in shampoos meant to be rinsed off as a preservative. Like many other long words, chemical names are built from smaller elements, and they aren’t hard to pronounce if you can recognize those smaller elements and break up the word: methyl-chloro-iso-thi-azol-in-one. (‘meth’ is pronounced to rhyme with ‘death’ and ‘yl’ as ‘ill’ and by North American chemists and as in ‘heath’ and ‘bile’ by chemists in the UK, Australia and New Zealand.) Chemical names often have a rhythm that makes them pleasant-sounding. This shampoo bottle is such a name, and I think my favorite is ethylenediaminetetraacetate (look for that one as EDTA on your mayonnaise* or salad dressing).
*: I once encountered someone who was demonstrating a product that could be used to make mayonnaise, and she was asking if anyone in the audience knew what EDTA was, because her product could make mayonnaise ‘without chemicals’. I said ‘ethylenediaminetetraacetic acid’, but I don’t think she was prepared for the possibility that someone would know that. The correct answer was ‘a chemical’. The strange thing is that it’s not just a chemical– it’s a preservative.
I refuse to believe that you used it “all the time”. How often do kids want to discuss 19th-century English theocratic policy? I certainly knew the word as a kid, and floccinaucinihilipilification, and pneumoultramicroscopicsilicovolcanoconiosis, but only from reading the Guinness Book of Records. I remember finding it odd how people would baulk at trying to pronounce them, when of course as Roche says it’s easy if you just break them into syllables.
Then, of course, I did a chemistry degree, where if you ignore the hyphens some of the chemical names could go on all day…
Ethylenediaminetetraacetic acid should strictly be called 2,2’,2",2’"-(ethane-1,2-diyldinitrilo)tetraethanoic acid, if my org chem hasn’t deserted me, but that is less catchy and I don’t know of any chemists that use the name. 
DDT is also quite mellifluous: dichlorodiphenyltrichloroethane.
Perhaps the longest scientific term which counts as ‘common’ is chlorofluorocarbon?
I doubt that “characteristically” would raise eyebrows in many conversations.
misinterpretation is pretty long - 17 letters - and fairly common.
And of course you can put “un” on the front, making an even 20 letters.
I vote for electroencephalograph.
Of course, I don’t know anyone who doesn’t just call it EDTA.
And “similes” beats it by one letter.
Uncharacteristically
20 letters, and not unnaturally cobbled together.
How about taut – there’s an AU between the Ts. (AU = astronomical unit, about 92 million miles or 150 million km).
As far as I can tell, the OP has no specific answer and this thread probably belongs in IMHO or GD or somewhere.
Smiles is wrong - the correct answer is actually beleaguered - it does contain an entire league.
How about hydrocarbofluorocarbons?
The longest one I can come up with (that isn’t a technical/scientific term or a word that’s mostly used as an example of a long word) is:
overcommercialization
21 letters
I thought Supercalifragilisticexpealidoucioius had a X in it? The top 10 of everything (Ash) mentions the top ten longest words. Super Calif is mentioned at the end of the list.
as an aside the following is also mentioned:
A Court case over who owns the rights to Super Calif ; Disney, or another “Average” person (Who wrote a song with it).
One of the first things the judge did: Required that the phrase “that word”
be said instead of Supercalifragilisticexpealidoucioius.
Meeko, I typed a guessed spelling into google and it came back “do you mean …?”. I used the spelling google gave me. And I’ve always pronounced the word with an S sound for that syllable. Your post is the first suggestion I’ve seen for a X.
That judge’s ruling is incredible. Supercalifragilisticespialidocious is not a obscene word or anything. What is the basis for prohibiting it? His goofy ruling sets up a cool slogan though… Freedom of speech is the right to say supercalifragilisticespialidocious in a courtroom!
Nice, although undercommercialization and hypercommercialization top it.