No, Antidisestablishmentarianism Is Not in the Dictionary
But we'll tell you the real "longest word"
So. What's the longest word in the dictionary?
If you said antidisestablishmentarianism, you're wrong. The longest word in Merriam-Webster's Collegiate Dictionary is currently electroencephalographically, which means "by using electroencephalography to examine a patient," as in, "Have you examined the patient electroencephalographically?"
In fact, antidisestablishmentarianism isn't entered in any of our dictionaries. Yes, we know: we enter electroencephalographically and don't enter antidisestablishmentarianism? What kind of lexicographers are we?
We are the kind that like to enter words with meanings. There are three criteria all words must meet in order to be entered into the dictionary: widespread usage, sustained usage, and meaningful usage; that is, evidence that the word is used to refer to something.
Where antidisestablishmentarianism gets disqualified first is the "meaningful usage" criterion. Go ahead: what does "antidisestablishmentarianism" mean? It's an example of a long word, but that's not its meaning. A word with meaning contributes essential information to a clause or sentence. When antidisestablishmentarianism is used as an example of a long word, it doesn't have a meaning at all.
But can't we construct a meaning for it? Sure. Based on its component parts, antidisestablishmentarianism would mean something like, "opposition to depriving a legally established state church of its status." Our problem is that we have scant evidence of it used with that meaning. In our files, which go back over 100 years, we only have three citations for antidisestablishmentarianism used with something like that meaning. Three citations do not constitute "sustained" or "widespread" use.
You could make a case that we should enter antidisestablishmentarianism simply because it's a well-known word. With more widespread, sustained, meaningful use, we will, and then it will be the longest word in our Collegiate Dictionary.
Unless we enter "supercalifragilisticexpialidocious."