The words in the list below are of a few basic types: chemical names, jargon terms of a particular specialised field, or nonce words made up for the occasion, and a jocular one at that.
The words that are most often quoted as the longest are antidisestablishmentarianism, floccinaucinihilipilification and supercalifragilisticexpialidocious. The first of these is only 28 letters long, but has the distinction of being a word which actually referred to something in the real, every-day world, and it is formed along the usual lines of English word formation. A disestablishmentarian was one who favoured the disestablishment of the state church. One who was against it was, naturally, an antidisestablishmentarianism. It seems to have occured to no one that a possible adverb formed from this word would be antidisestablishmentarianistically (34 letters). In any case the plural of it would be 29 letters long, making it just as long as the next word.
Floccinaucinihilipilification (29 letters), is famous for being the longest word in the Oxford English Dictionary, however, this is now only true of the first edition, the second edition contains a few even-longer words. The word itself was a joke word of the 18th and 19th century literati, stemming from an Eton grammar school Latin drill.
Supercalifragilisticexpialidocious (34 letters) was made famous by the Walt Disney movie 'Mary Poppins' (1964), though it has been recorded earlier, going way back to the 1940s. Once again a joke word, it first appeared in comedy songs, of which the humour was supplied by the ridiculousness of the word. Nowadays it is used (also in a shortened version supercalifragilistic) to mean 'amazingly excellent', etc.
One sly answer to the question of what is the longest word would be DDT, for its full title is dichlorodiphenyltrichloroethane (31 letters). This is longer than antidis... and flocci... and is more likely to be considered a 'real' word than the slightly longer supercali... Also, three letters are easier to remember than 31. It is the longest in the Macquarie Dictionary.
Many of these long words were collected by Josefa Heifetz Byrne in her wonderful dictionary Mrs Byrne's Dictionary of Unusual, Obscure and Preposterous Words (1974).
The longest word to appear in standard English dictionaries is pneumonoultramicroscopicsilicovolcanokoniosis. This is the name of a lung disease suffered by miners. It first appeared in Webster's New International Dictionary and then alter in the second edition of the Oxford English Dictionary.
humuhumunukunukuapuaa - 21
dihydroxyphenylalanine - 22
hexamethylenetetramine - 22
pseudomonocotyledonous - 22
honorificabilitudinity - 22
interdenominationalism - 22
gynotikolobomassophile - 22
bathysiderodromophobia - 22
dichlorodifluoromethane - 23
polytetrafluoroethylene - 23
quasihemidemisemiquaver - 23
inanthropomorphisability - 24
sphragidonychargokometes - 24 (Ancient Greek)
pseudorhombicuboctahedron - 25
honorificabilitudintatibus - 26 (longest in Shakespeare)
antidisestablishmentarianism - 28
tetramethyldiamidobenzhydrols - 29
floccinaucinihilipilification - 29 (longest in OED 1st ed.)
hippopotomonstrosesquipedalian - 30 (Mrs Byrne's Dict.)
dichlorodiphenyltrichloroethane - 31 (longest in Macquarie Dict.)
praetertranssubstantionalistically - 34 (M McShane)
supercalifragilisticexpialidocious - 34
necropurogeohydrocheirocoscinomancy - 35 (Thomas Tomkis)
cholangiocholecystocholedochectomies - 36 (Mrs Byrne's Dict.)
hepaticocholangiocholecystenterostomies - 39 (Gould's Medical Dict.)
osteoarch‘matosplanchnochondroneuromuelous- 42 (TL Peacock)
pneumonoultramicroscopicsilicovolcanokoniosis - 45 (longest in OED 2nd ed. & Websters)
antipericatametaanaparcircumvolutiorectumgustpoops - 50 (Rabelais)
aequeosalinocalcalinoceraceoaluminosocupreovitriolic - 52 (Mrs Byrne's Dict.; chem)
osseocarnisanguineoviscericartilagninonervomedullary - 52 (TL Peacock)
bababadalgharaghtakamminarronnkonnbronntonnerronntuonnthunntrovarr hounawnskawntoohoohoordenenthurnuk - 100 (James Joyce)
- 310 (not a real word - see appendix 2)
- 1,185 (chem. in Mrs Byrne's Dict. See appendix 2)
- 1,913 (chem. in Mrs Byrne's Dict. See appendix 2)
``THE long word of 310 letters was used as a means of demonstrating: 1. The extent to which even the English language is capable of forming enormous word monsters, and, 2. The whole field of superstitious divinatory practices which are as old as humanity. The literal translation of the long word is "A deluded human who practices divination or forecasting by means of phenomena, interpretation of acts or other manifestations related to the following animate or inanimate objects and appearances: birds, oracles, Bible, ghosts, crystal gazing, shadows, air appearances, birth stars, meteors, winds, sacrificial appearances, entrails of humans and fishes, fire, red-hot irons, altar smoke, mice, barley, salt, lead, dice, arrows, hatchet balance, sieve, ring suspension, random dots, precious stones, pebbles, pebble heaps, mirrors, ash writing, dreams, palmistry, nail rays, finger rings, numbers, book passages, name letterings, laughing manners, ventriloquism, circle walking, wax, susceptibility to hidden springs, wine and shoulder blades. Various monastic author of the Middle Ages writing on the subject of human superstition have actually used such a long word with a slightly varying sequence of items.''
It is clearly NOT a word of the 'Middle Ages' since it uses word elements that do not date back that far, for example the '-bletono-' refers to Bletonism a word which first appeared in the C19th. I assume that Ripley merely concocted it himself. It is merely a list of the first elements of a set words ending in -mancy (eg theomancy, bibliomancy, etc.) all stuck together in one long line. Furthermore it plainly is derived from the set of such words that appeared in Roget's Thesaurus (under the head prediction 511), for the order of the elements follows exactly non-alphabetical ordering of Roget's. One assumes that the terminal element -naniac is a mistake. Presumably it should be a derivative of -mancy (the usual occupational agent termination being -mancer). Possibly -maniac was meant, it being etymologically related.
These last two words are respectively, the chemical name of the protein part of the tobacco mosaic virus (C785H1220N212O248S2), and the chemical name for tryptophan synthetase A protein. These appear in Mrs Byrne's Dictionary of Unusual, Obscure and Preposterous Words.
The Thomas Love Peacock words are from Headlong Hall 1816 [published (along with Nightmare Abbey) in the 'World's Classics' series, OUP, 1929: xi. 90-1]
The Thomas Tomkis word is from his 1615 play Albumazar.
The Shakespearian word comes from Love's Labour's Lost V.i. Not a coinage of Shakespeare's though. It is the ablative plural of the Latin honorificabilitudintas, which is an extension of honorificabilis meaning 'honorableness'. It first occurs in English in 1599, used by Thomas Nashe.
The Mark McShane word is from his novel Untimely Ripped 1963.
The Rabelaisian word appears in Gargantua and Pantagruel occurring there as a title of a book "Antipericatametaanaparcircumvolutiorectumgustpoops of the Coprofied".
The Joycean concoction is, not surprisingly, from Finnegan's Wake. it is meant to represent the thunder-clap that accompanied Adam & Eve in the Fall. Other 100-letter long words appear in the book.