The longest word in ENGLISH!

The longest words in Oxford Dictionaries Online are:

* antidisestablishmentarianism opposition to the disestablishment of the Church of England - 28 letters
* floccinaucinihilipilification the estimation of something as worthless - 29 letters
* pneumonoultramicroscopicsilicovolcanoconiosis a supposed lung disease - 45 letters

You're unlikely to come across these words in genuine use: they're
generally just provided as answers to questions about the longest words
in the English language. In terms of sheer size, however, the longest
word to be found in Britain is the Welsh place name Llanfairpwllgwyngyllgogerychwyrndrobwllllantysilio gogogoch. For obvious reasons, it's usually abbreviated to Llanfair PG.

The 20-volume historical Oxford English Dictionary includes other very
long words, most of which are highly technical. These include:
* otorhinolaryngological - 22 letters
* immunoelectrophoretically - 25 letters
* psychophysicotherapeutics - 25 letters
* thyroparathyroidectomized - 25 letters
* pneumoencephalographically - 26 letters
* radioimmunoelectrophoresis - 26 letters
* psychoneuroendocrinological - 27 letters
* hepaticocholangiogastrostomy - 28 letters
* spectrophotofluorometrically - 28 letters
* pseudopseudohypoparathyroidism - 30 letters