supercalifragilisticexpialidocious
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.

Taken from the OED.