It's a word protesting how the English language comes up with words. You say it in situations that would be also appropriate for crazy offbeat words such as supercalifragilisticexpialidocious. The origin comes from the catastrophe of having the word impPROvise and not imCONvise and then it grew having a ridiculous number of prefixes and suffixes added on. Proper pronuniciation is fast but with a pause before the "ness".
Cat's got my tongue! UNIMCONVISATIONALISTICISMNESS! Arrrgh!