It's a nonsense word made famous from a song of the same name in the 1964 Mary Poppins film.
It has an entry in the Oxford English Dictionary:
A nonsense word, originally used esp. by children, and typically expressing excited approbation: fantastic, fabulous.
The first mention before the film is the title of another song, Supercalafajalistickespialadojus, written by G. Parker & B. Young in 1949. Parker and Young attempted to sue the film-makers for copyright infringement, but, according to Wikipedia:
The Disney publishers won the lawsuit partially because affidavits were produced showing that "variants of the word were known ... many years prior to 1949".