No, Antidisestablishmentarianism Is Not in the Dictionary
So. What's the longest word in the dictionary?
If you said antidisestablishmentarianism, you're wrong. The longest word in the Merriam-Webster.com dictionary is currently acrylonitrile-butadiene-styrene, which refers to "a tough rigid plastic used especially for automobile parts and building materials," also helpfully known as ABS.
In fact, antidisestablishmentarianism isn't entered in any of our dictionaries. Yes, we know: we enter acrylonitrile-butadiene-styrene 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 being 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 the Merriam-Webster.com dictionary.
Unless we enter "supercalifragilisticexpialidocious."