The word antidisestablishmentarianism, with 28 letters, is the longest accepted word in the English language. There are other words, however, that are longer. For example: Supercalifragalisticexpialadocious is a nonsense word meaning fantastic. However, since it was taken from Mary Poppins, a fantasy novel for children, it is not as yet considered to be a real "word." The claim that "antidisestablishmentarianism" is the longest word in the English language is actually quite arguable, since antidisestablishmentarianism is used seriously in academic and ecclesiastical writing about the Church of England when the concept arises, which it does occasionally (See e.g. Hastings and "Some notes on the Church of England and Establishment", below). However, since 1992 it has lost this title to floccinaucinihilipilification in the Guinness Book of Records. (see also: Longest word in English)
If the word is taken literally, it could mean 'against the enemies of the establishment, but not necessarily aligned with the establishment'.
Antidisestablishmentarianism might still be the longest word not invented for the specific purpose of being a long word, however. (the word floccinaucinihilipilification may have been invented as a joke, and is an amalgamation of four Latin words). Presumably its creation invoked the irony of its length, and may have to do with the politics and commentary of the time period. Also, some criticise its validity on the basis that it has two prefixes ("anti-" and "dis"-) and three suffixes ("-ment", "-arian" and "-ism"), stating that it is not only possible to create a number of longer meaningful words by adding ever more prefixes, but that "antidisestablishmentarianism" is not a standalone word because of them. For example, theoretically "antiantidisestablishmentarianism" is a valid word, where it refers specifically to antidisestablishmentarians' opponents, rather than merely disestablishmentarians in general.
If you take terrorism to be against the establishment, and thus disestablishmentarian, then those who oppose terrorism (but don't necessarily support the target of terrorism) could be said to be displaying antidisestablishmentarionism.
In Popular Fiction
In the Honeymooners episode, The $99,000 Answer, Alice asks Ralph to spell the word. Ralph says he'll spell it if she gives him $16,000 for spelling it. She tells him she'll give him twice the amount if he can say it.
In the Blackadder the Third episode, Dish and Dishonesty, the title character tells Prince George that he shall return 'before you can say antidisestablishmentarianism'. The Prince makes many attempts at saying it, eventually reducing to the level of saying anti-distinctly-minty, by which time Blackadder has returned two days later.
In Aqua Teen Hunger Force The Cybernetic Ghost of Christmas from the Future makes the statement that tacos do not exist in the future, stating, "I think we all know why. Anti taco legislation! Disestablishmentarianism!"