a song in the play Chitty Chitty bang bang
The word (in song) from Mary Poppins (1964) is spelled "SUPERCALIFRAGILISTICEXPIALIDOCIOUS". (see related question)
That is not a word, really. The longest word that most people reference is "hyperpolysyllabicsesquipedalianism", which is longer, anyway. By the way, this word means 'the extreme practice of using long words'.