One of the classic long words is floccinaucinihilipilification, which means estimating something as worthless. It was coined by students at Eton, combining a number of roughly synonymous Latin stems. Latin flocci, from floccus, a wisp or piece of wool + nauci, from naucum, a trifle + nihili, from nihilum, nothing + pili, from pilus, a hair, something insignificant (all therefore having the sense of “worthless” or “nothing”) + -fication.
At 29 letters, the word has many instances of use and appears in many larger dictionaries, such as the Oxford English Dictionary.
It is one of the longest non-technical terms with many instances of printed use. Its first use in literature was in 1741 in William Shenstone’s Works in Prose and Verse: “I loved him for nothing so much as his flocci-nauci-nihili-pili-fication of money”.