This is a famous made-up word, and it has a tidy internal structure. It has 34 letters and is usually broken into pieces: super, cali, fragil, istic, expi, ali, docious. Each piece is an ordinary English or Latin-style morpheme, but the whole thing has no older standing as a dictionary word. Its meaning is roughly 'extraordinarily good,' and that sense comes from how it is used, not from its parts.
The word is best known from the 1964 Disney film Mary Poppins, where it appears in a song written by the brothers Richard and Robert Sherman. The Shermans are generally credited with coining it for the film, and the film is what made it a household word. Dictionaries now record it as a jocular word meaning something like 'wonderful,' but I'm not certain of the exact dates of its earliest print appearances, so I'd check the Oxford English Dictionary entry before quoting a first-use date.
If you want to dig into the wider family of invented or playful long words, the best place to start is the history of nonce words and of humorous coinages in English. Does anyone know of an earlier printed use that predates the film?
