pneumonoultramicroscopicsilicovolcanoconiosis
dictionary PNEUMONOULTRAMICROSCOPICSILICOVOLCANOCONIOSIS (45 letters; a lung disease caused by breathing in certain particles) is the longest word in any English-language dictionary. (It is also spelled -koniosis.) 020214
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(useless?) information On Feb. 23, 1935, the New York Herald-Tribune reported on page 3:

Pneumonoultramicroscopicsilicovolcanokoniosis succeeded electrophotomicrographically as the longest word in the English language recognized by the National Puzzlers' League at the opening session of the organization's 103d semi-annual meeting held yesterday at the Hotel New Yorker.
The puzzlers explained that the forty-five-letter word is the name of a special form of silicosis caused by ultra-microscopic particles of siliceous volcanic dust.

The word appears in the 1936 Supplement to OED1, the OED2, the addendum to W2 (spelled -koniosis), W3 (spelled -coniosis), RHUD2, and Chambers.
The OED2 has:

pneumonoultramicroscopicsilicovolcanoconiosis (-koniosis), a factitious word alleged to mean 'a lung disease caused by the inhalation of very fine silica dust' but occurring chiefly as an instance of a very long word.
1936 F. Scully Bedside Manna 87 *Pneumonoultramicroscopicsilicovolcanakoniosis [sic], a disease caused by ultra-microscopic particles of sandy volcanic dust, might give even him laryngitis.
1966 Word Study Oct. 7/2 The resources of Greek have enriched the modern world as well as the ancient one. Perhaps this is most dramatically illustrated by the longest and most fantastic word now in an English dictionary (the Merriam-Websterís great Unabridged) which is forty-five letters in length: pneumonoultramicroscopicsilicovolcanoconiosis,..meaning 'a disease of the lungs caused by extremely small particles of ash and dust'.
1973 R. Megarry Second Miscellany-at-Law 160 It has been said that 'floccinaucinihilipilification' is the longest word in the English language... The wordís proud title must yield to some technical terms, such as pneumonoultramicroscopicsilicovolcanokoniosis.
The following appeared in a post in alt.usage.english:
I conjecture that this "word" was coined by word puzzlers, who then worked assiduously to get it into the major unabridged dictionaries (perhaps with a wink from the editors?) to put an end to the endless squabbling about what is the longest word.
Karl F. Lingenfelder reports that the domain name pneumonoultramicroscopicsilicovolcanoconiosis.com was registered on October 28, 1999, and that when activated it will point to http://www.mauigateway.com/~team/longestwordinenglish/ This is a commercial website selling domain names.
020214
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852456 antidisestablishmentarianism
hyperpolysyllabicsesquipedalianist

food for thought
030315
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TurdFerguson I had to write this word 100 times because I got in trouble in computer class 030315
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lani aand jesse yup... just as i supected 040406
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lani and jesse oops 040406
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kookaburra the word for the phobia of long words is Hippopotomonstrosesquippedaliophobia.

thats somemore to think about
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dudeinanigloo *stares at screen, blinks, scratches head*

My brain hurts...
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magdalena Thank you dictionary..
I feel much more enlightended now...
I never thought to read one myself..
Instead I prefer to waste my time getting drunk and talking to all these boring ass people on blather...

hahahahahaha.... yeah ....whatever...
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minnesota_chris it's also a word that was created for the express purpose of being the longest word. It isn't really used. Just like floccinaucinihilipilification, which was also created to annoy functional-based linguists like me. 040521
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lauren. that's just not true, i use it all the time on english essays. 050125
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u24 shit, kookaburra, it really is. I didn't believe you so I checked on dictionary.com.

wow.
050126
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Smat Lolz long word 060504
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Seraphmodel.03 ibelieveitiscausedddd

by breathing in fine quartz dust

i knew that from my general knowledge, how sad am I :S

also from my general knowledge, i know that a standard gold ball has 365 indentations on its surface
060504
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blather spell check golf 060504
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epitome of incomprehensibility I wouldn't mind a golf ball made of gold, myself. Give me one and I'll make up a 50-letter word for it. 060505
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