From the Oxford English Dictionary
supercali
fragilistic
expiali
docious, a.
[Fanciful: cf. SUPER a. 3.]
A nonsense-word used esp. by children, now chiefly expressing excited approbation: fantastic, fabulous.
Made popular by the Walt Disney film ‘Mary Poppins’ in 1964. The song containing the word was the subject of a copyright infringement suit brought in 1965 against the makers of the film by Life Music Co. and two song-writers: cf. quots. 1949, 1951. In view of earlier oral uses of the word sworn to in affidavits and dissimilarity between the songs the judge ruled against the plaintiffs.
1949 PARKER & YOUNG (unpublished song-title) Supercalafajalistickespialadojus. 1951
(song-title) Supercalafajalistickespeealadojus; or, The super song. 1964 R. M. & R. B. SHERMAN (song-title) Supercalifragilisticexpialidocious! 1967 Decisions U.S. Courts involving Copyright 1965-66 488 The complaint alleges copyright infringement of plaintiff's song ‘Supercalafajalistickespeealadojus’ by defendants' song ‘Supercalifragilisticexpialidocious’. (All variants of this tongue twister will hereinafter be referred to collectively as ‘the word’.) 1971 Daily Tel. 6 Nov. 13/5 If you can stand more than a day of Supercalifragilisticexpialidocious entertainment you can settle in at the concrete Contemporary Resort Hotel. 1972 Atlanta Constitution 9 Apr. 20A/1 Disney World, the new supercalifragilisticexpialidocious tourist attraction created by the folks who brought you Mickey Mouse, is packing them into Orlando. 1980 Amer. Speech LV. 266 Whatever the ancestry of supercalifragilisticexpialidocious, it entered the general public consciousness as a result of the wonderful world of Disney. 1982 N.Y. Post 29 July 64/3 His eyes are willing unspoken words to life as though they were part of one of those supercalafragilistic electronic scoreboards.