The only dictionary you will find this in is the one that Mary Poppins carries around in her handbag.

Supercalifragilisticexpialidocious is a word dreamt up for the purposes of the movie's score. Until that point the word hadn't existed and probably wouldn't have been invented otherwise.

This 34 letter word hasn't much featured in hard copy dictionaries, but it is well defined, so to speak, on a variety of online dictionaries. According to Allwords.com, supercalifragilisticexpialidocious is an adjective meaning
absolutely stunningly fantastic.

According to one learned commentator on the English language who has broken the word down into its component parts, the real meaning is "Atoning for extreme and delicate beauty while still being highly educable.

The word, one of the longest in the English language, entered common useage through the 1964 Walt Disney movie starring Julie Andrews. A music company and two songwriters unsuccessfully took on Disney in the courts and lost. They claimed to have used the word as early as 1949.