The Longest word may have once been,
Supercalifragilisticexpialidocious at 34 letters.
Though it is considered a "coined" term & not a true 'word'.
The longest word you might actually run across may be
Pseudopseudohypoparathyroidism (30) which is a genetic disease which causes a person to respond abnormally to certain hormones.
is the longest know word in Greek, translated into English.
This might not be considered a 'real word' either, due to the way it was formed by stringing the names of many foodstuffs together.
If you really want to get crazy, we have to look at chemical compound names. Of course, these are like the Greek word above, a string of compounds.
So there is no need to paste here to see, yet know they can be many hundreds of letters long.
It goes on & on.....& on some more.
I think the funniest I have found is the Native American word for what is known as Lake Webster, Massachusetts, today.
Chargoggagoggmanchauggagoggchaubunagungamaugg @45 letters, means..."you fish your side of the water, I fish my side of the water, nobody fishes the middle".