The word antidisestablishmentarianism grew to its immense length as religious and political events unfolded in England. In the early 16th century, there was much discontent in England with the Roman Catholic Church. (This is right on the heels of Martin Luther nailing his famous 95 theses to the door of the church in Wittenberg in 1517, thereby setting off the Protestant Reformation.) Conflicts of authority between King Henry VIII and Popes Clement VII (1523–1534) and Paul III (1534–1549) led Henry VIII to split from the Roman Catholic Church and form the Church of England, known in the rest of the world as the Anglican Church.
What is antidisestablishmentarianism?
At this time, the Church of England became the official church of the state. Those who favored the establishment of the state church were establishmentarians, and the movement was called establishmentarianism.
During the 19th century, perhaps following the precedent set in the United States, many wanted a clean separation between church and state and pushed to remove the Church of England's status as the official state church. These people favored the disestablishment of the Church of England, so their movement was called disestablishmentarianism.
In opposition to those who wanted to disestablish the church, the antidisestablishmentarians lobbied to keep the Church of England official. Antidisestablishmentarianism won out in England, and to this day, the monarch of Great Britain is also the "Supreme Governor of the Church of England." The movement eventually failed, however, in Ireland and Wales.
Some people argue that antidisestablishmentarianism isn't a "real" word because it uses two prefixes (anti- and dis-) that cancel each other out. The name for the movement, they say, should revert to the original establishmentarianism. Indeed, it could be said that the opposite of disestablishmentarianism is both antidisestablishmentarianism and establishmentarianism, so they mean approximately the same thing. The subtle difference between the two, though, is that the longer word denotes opposition to a particular movement — in this case, disestablishmentarianism — whereas the shorter word only indicates support for a state church.