I come to this word in the hope that the piece you are about to read won’t be an example of it. Perissology means using more words than necessary to explain one’s meaning, a pleonasm. Since perissology is three letters longer than pleonasm but means the same, you may argue it’s an example of the related habit of using long words when shorter ones will do.
The word comes to us from the post-classical Latin of the fourth or fifth century AD. Romans of classical times knew it as a Greek word, perissologia, which came from perissos, beyond the usual number or size, redundant, superfluous. The prefix perisso- is known in two other very uncommon English words: perissosyllabic, a line of verse that has more syllables than normal, and perissodactyl, a grazing mammal with hooves made up of an odd number of toes, which sounds obscure but is a characteristic of horses as well as tapirs and rhinoceroses. Its opposite is artiodactyl, having an even number of toes, which refers to mammals such as pigs, deer, goats and cattle.
Perissology came into English at the end of the sixteenth century but was never anything more than an obscure literary word. In recent centuries it has mainly been exploited for humorous effect.
His inscience of avitous justicements, and of lexicology, his perissology and battology, imparted to his tractation of his cause, an imperspicuity which rendered it immomentous to the jurator audients.
Letters to Squire Pedant, by Samuel Klinefelter Hoshour, 1856. This described a lawyer pleading his case. It says that his knowledge of old judgements and the nature of words, plus his unnecessary repetition, made his case so obscure the jury decided it was unimportant. Battology is another word for perissology; hair-splitting scholars find a distinction between battology, perissology and pleonasm, but we may let that pass us by.