Floccinaucinihilipilification is rarely used. I don’t think I need to explain why.
It means ‘the action or habit of believing something to have no value’.
The use of floccinaucinihilipilification is so unusual that it made the news in 2012 when a British MP spoke the word in the House of Commons. It became the longest word in Hansard (the official report of proceedings).
You can listen to the British English pronunciation here if you fancy trying it out yourself.