At over 180,000 letters long, the chemical name of the protein titin is often said to technically be the longest English word. If spoken out loud, this word takes over three hours to say! Its absurd length is due to the fact that proteins get their scientific names by combining the names of all of their joined amino acids together, and titin has quite a lot of them. For obvious reasons, titin’s official name has never actually appeared in a dictionary or scientific text. Because it is a scientific term, many would disqualify the Big M from actually taking the crown as English’s longest word.
Antidisestablishmentarianism
Supercalifragilisticexpialidocious
I spelled this from heart
Also,
https://www.dictionary.com/e/longest-english-words/
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch