"Supercalifragilisticexpialidocious" is the 34-lettered song title from the 1964 movie Mary Poppins. As a song title, it is a proper noun, but the word, and variations, has entered the English language as an adjective. It is one of the longest words in the English language.
The song describes using the word as a miraculous way to talk oneself out of difficult situations, and even as a way to change one's life. The song appears in the film's animated sequence where Mary Poppins is harangued by reporters after winning a horse race and responds to one claiming there are not words to describe her feelings of the moment. Mary disagrees with that and begins the song about one word she can use.
The word itself has obscure origins, pertaining as to when it was first used, but the roots are fairly clear, as Richard Lederer wrote in his book Crazy English: super- "above," cali- "beauty," fragilistic- "delicate," expiali- "to atone," and docious- "educable," the sum meaning roughly "Atoning for extreme and delicate beauty while still being highly educable."