The word “supercalifragilisticexpialidocious” seems to pre-date Mary Poppins, but language experts have yet to pin down by how much, or what exactly, it originally meant. Its use in the movie may have been inspired by a nonsense word the Sherman brothers remembered.
According to Richard M. Sherman, co-writer of the song with his brother, Robert, the word was one that the two knew in their youth. In a 2007 interview, Sherman indicated that the final version of the word was produced by the two brothers over the course of two weeks during the songwriting process, indicating only that the origins of the word were in their memories of creating double-talk words in their childhood.
The roots of the word have been defined as follows: super- “above”, cali- “beauty”, fragilistic- “delicate”, expiali- “to atone”, and -docious “educable”, with the sum of these parts signifying roughly “Atoning for educability through delicate beauty.” According to the film, it is defined as “something to say when you have nothing to say”.
‘Supercalifragilisticexpialidocious’ was first added to the Oxford English Dictionary in 1986. – Wikipedia