FLOK-sih-noh-see-NEE-hee-lee-PEE-lih-fih-KAY-shun

The Oxford English Dictionary classifies it as 'humorous' and with 29 letters it's one of the longest words in the English language¹. Moreover, unlike pneumonoultramicroscopicsilicovolcanoconiosis² and the actual longest word, which are highly scientific and specific, floccinaucinihilipilification can be used in everyday speech:

I loved him for nothing so much as his floccinaucinihilipilification of money.
- William Shenstone, Letters

They must be taken with an air of contempt, a floccinaucinihilipilification of all that can gratify the outward man.
- Sir Walter Scott, Journal

I note your distress at my floccinaucinihilipilification of the CTBT.
- US Senator, Jesse Helms, referring to the Comprehensive Test Ban Treaty

Yes, my apparent floccinaucinihilipilification of both you and your canine companion was meant to be stupendously supercilious.
- Any Sesquipedalian Person³

But what does this strange and gigantic word mean? It's actually an aggregate of Latin forms.

• 'flocci' is from the Latin floccus, which means 'fleece' and is related to the verb floccipendo which means, literally, 'to give the value of a bit of fleece' or 'to take lightly'.

• 'nauci' is a word meaning 'few' or 'almost nothing'.

• 'nihil', like in annihilation, is the Latin word for 'nothing'.

• 'pili' is the plural form of the word pilus, the Latin word for 'small hair', but in this case meaning 'trifle'.

• 'fica' is from the verb facio meaning 'to do' or 'to make'.

• 'tion' is a standard English nominalisation form.

When put together, we get 'the making light of a few trifles of nothing'. Thus, the meaning of floccinaucinihilipilification becomes 'the act of estimating something as worthless'. Of course, the word can then be edited to form verbs, like floccinaucinihilipilificate, and adjectives, like floccinaucinihilipilificatious, or even other nouns, like floccinaucinihilipilificatism.

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¹ It's actually one letter longer than the next most commonly referred to long word, antidisestablishmentarianism, the ideology of being against the dissolution of the Church of England.
² This is defined as a lung disease caused by the inhalation of very fine silica dust.
³ Literally, this is any person who uses words a foot-and-a-half long.