Supercalifragilisticexpialidocious is a very long nonsense word meaning "fabulous" or "splendiferous." It became well-known after the release of the 1964 film Mary Poppins, and as the tune says, "If you say it loud enough, you'll always sound precocious."
Supercalifragilisticexpialidocious was made popular by the movie and its soundtrack, which includes a song that manages to rhyme the cumbersome word with both "atrocious" and "precocious." But the invented adjective was already being used in 1930s slang to describe something grand or wonderful. Sometimes the 34-letter supercalifragilisticexpialidocious is incorrectly cited as the longest word in English, but according to most dictionaries, that honor actually belongs to a 45-letter term for a lung disease caused by inhaling volcanic dust.