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.)
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-Webster's 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 word's proud title must yield to some technical terms, such as pneumonoultramicroscopicsilicovolcanokoniosis.
