The Oxford English Dictionary estimates the word was first used in the 1940s. According to Wikipedia, columnist Helen Herman muses about her made up word, Supercaliflawjalisticexpialidoshus, in her A-Muse-ings column on March 10, 1931.
The brothers admitted that the word was one they knew from their youth. Richard Sherman stated the origins of the word was in their memory, but they finalized the word in about two weeks of the song writing process. To them the word created memories of doubletalk from their younger days.
In interviews about The Sound of Music, Julie Andrews and the actors that played the seven children tell stories about her teaching them to sing the song Supercalifragilisticexpialidocious, as well as to say the word backwards. Mary Poppins had not been released at the time The Sound of Music was being filmed.
In 1986, Supercalifragilisticexpialidocious was added to the Oxford English Dictionary.
The song ranked #36 in AFI’s 100 Years…100 Songs.