The word has to be in the Dictionary, it can’t be one you have made up yourself.
I’ll start it off with this one: Floccinaucinihilipilification
Supercalifragilisticexpeialidocious of course.
Oh, dictionary words… Let’s search
aequeosalinocalcalinoceraceoaluminosocupreovitriolic (52 letters)
With no help from the internet i can come up with:
firefighterlabel, slashdotted, convertibles, approbation.
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch (Town in Wales UK)
this thread is kind of dead now, Norty69er said the longest word
What’s the longest word you know.
Dumbass
longest I know:
pneumonoultramicroscopicsilicovolcanoconiosis.
longest I can pronounce:
antidisestablishmentarianism
Chinese words can be as long as one can get tired. Long words are made up of multiple words and the difference between a long word and a short word is whether there’s a space or not. They don’t need to be in the dictionary.
Kenshin you should be a philosopher and a book writer
Internet 
, three syllables and I’m still learning. 
mailbox… coz it has lots of letters in it. 
antidisastablishmentarialism
is the longest word i know don’t have a clue what it means but, it is accordingly the longest word in the dictionary
Pseudoantidisestablishmentarianism, false opposition to the separation of the state and the church.
I know. I didn’t really “know it”. I looked it up 
u would have to go one better lol
i just started a thread on AMD and PENTIUM if anyone is intested
elastic
because it stretches :o
thats a gd one i like that
the old ones usualy are the best 
World’s longest geographical name, a mountain in New-Zeland:
Taumatawhakatangihangakoauauotamateaturi- pukakapikimaungahoronukupokaiwhenuakitanatahu
i think you must win it with that
one problem though can you say it all in one go
how about this for the longest word in a song:
Taumatawhakatangihangakoayauo-tamateaturipukakapikimaungahoro-nukypokaiwhenuakitanatahu
BY Quantum jump - The Lone Ranger
But in theory that is three words
but still better than what i did by miles