"Supercalifragilisticexpialidocious" is a long word that was invented simply to be a long word. Its meaning is incidental. Nonetheless, it appears in some dictionaries. Wordsmith.org defines it as:

A nonsense-word used esp. by children, now chiefly expressing excited approbation: fantastic, fabulous.

(Or in other words, the last 29 letters are essentially superfluous.)

Most of us know this quite atrocious-sounding word from the song of the same name, which featured prominently in the 1964 Disney classic Mary Poppins. I always assumed that this was how "supercali..." was introduced into the English lexicon, and that Disney's fantastic songwriters Richard and Robert Sherman coined the word themselves. But it turns out that's only true for this specific spelling of the word.

The "supercali..." phenom was not a Sherman brothers invention. In fact, in 1965, Disney and the Sherman brothers faced a lawsuit from the writers of "Supercalafajalistickespeealadojus," a different song published in 1951 — 13 years before the Disney hit. They weren't technically the same word, but only a moron would think they weren't related. However, the lawsuit was dismissed, since it was determined that the word in question — or at least one similar enough — existed before either of the songs were written. (Also, Disney's song and the earlier song had nothing in common other than the word in question.)

So where did The Word come from in the first place? It's still a mystery. While it seems pretty clear that The Word was made up by someone — probably a kid — with the specific intention of creating "the longest word ever," there's no written documentation of its origin. Rumours persist that 19th century Scottish miners used the word to give "orders" to prostitutes, but that's almost certainly poppycock.

The Sherman brothers claimed it was coined in 1918 as "super-cadja-flawjalistic-espealedojus," and that they picked it up at summer camp in the '30s. But they couldn't document it either.

Sources: Wordsmith.org, The Straight Dope, The Phrase Finder