It's a silly word made famous by a song in the movie "Mary Poppins." No one actually uses it, and it's not quite the longest word. But it is fun to sing the song.
The longest word you'll find in a dictionary is pneumonoultramicroscopicsilicovolcanoconiosis (a lung disease). There are longer words, but experts don't agree on them being "words" or "English." 🤣