1.Hippopotomonstrosesquippedaliophobia (36 letters)
Ironically, Hippopotomonstrosesquippedaliophobia is one of the longest work in the dictionary and is the name for a fear of long words! Who would have thought, right?
2.Supercalifragilisticexpialidocious (34 letters)
Perhaps the best word of all! Made popular by the film Mary Poppins, Supercalifragilisticexpialidocious is something to say when you have nothing to say.
3. Pseudopseudohypoparathyroidism (30 letters)
A relatively mild form of pseudohypoparathyroidism that is characterized by normal levels of calcium and phosphorus in the blood.
4.Floccinaucinihilipilification (29 letters)
The longest unchallenged nontechnical word that not all directories recognize, that including Merriam-Webster. According to alternative sources, floccinaucinihilipilification is the act or habit of describing or regarding something as unimportant, of having no value or being worthless. Often times, it is used in a humorous way.
5.Antidisestablishmentarianism (28 letters)
A term referring to a political movement in 19th century Britain that sought to separate church and state. In this case, political movement wanted to disestablish the Church of England as the official state church of England, Ireland and Wales.
Interesting article