The Answer:

Ah, a trick question.

As viewers of Mary Poppins know, "supercalifragilisticexpialidocious" is a word that's... that's... well, it's supercalifragilisticexpialidocious. And there's a song about it. And in that song, Mary Poppins says that you could also say it backward, and does so. But how she does so is different between the film and stage versions.

In the recent stage musical, which opened in 2004 in London, and in 2006 on Broadway, Mary does what you might expect: she pronounces the word the way one would if its spelling were reversed. The backward version is suoicodilaipxecitsiligarfilacrepus.

In the 1964 film, however, only the syllables are reversed, with the exception of the first/last syllable. So, super-cali-fragil-istic-expi-ali-docious becomes docious-ali-expi-istic-fragil-cali-rupus. Our guess is that "rupus" was used instead of "super" because it sounded better.

The word does not appear in any of the Mary Poppins books by P.L. Travers.

—The Fact Monster