Bill Cipher is the mischievous dream demon who antagonises Dipper and Mabel in Disney's animated series Gravity Falls, while supercalifragilisticexpialidocious is the famously long nonsense word from Mary Poppins. Putting the word in Bill's playful, villainous voice turns a familiar musical reference into a piece of cartoon chaos, which is why short-form creators keep reusing it. It works as an in-joke for Gravity Falls fans and as a standalone tongue-twister for viewers who simply enjoy hearing an impossibly long word delivered with total confidence.