In the Song It means great, fantastic, extremely cool. It can also be used as an exclamation, or as a way to describe something that surprises and delights you, similar to the words 'stupendous' or 'wonderful'.
It is from the song in "Mary Poppins"(1964), suggested as "something to say when you don't know what to say."
The word is a neologism, meaning it was created (for the movie), but variations existed as early as 1949.
(See also the related question)
Root Meanings A semi-serious analysis of each part of the word can produce the alternate definition, which roughly means -
Atoning for educability through delicate beauty.
The roots of the word have been defined as follows:
- super- "above"
- cali- "beauty"
- fragilistic- "delicate"
- expiali- "to atone"
- docious- "educable"