Parseltongue, the language of snakes, has associations in the common mind with Dark Magic (although Dumbledore stated that it is not necessarily an evil quality). Those possessing the ability to speak it (“Parselmouths”) occur very rarely. People apparently acquire the skill through learning or via a method of xenoglossia, such as through genetic inheritance (or by use of Dark or dangerous Magic). Harry is a Parselmouth: Harry Potter and the Chamber of Secrets explains this as due to Voldemort’s passing on some of his abilities to Harry the night he tried to kill him. Deathly Hallows reveals that a part of Voldemort’s soul within Harry grants him this ability, which is later destroyed leaving Harry stripped of the ability.
Other known Parselmouths include Salazar Slytherin and his descendants, including the Gaunts and Voldemort. Dumbledore can also understand Parseltongue; however, he learnt it and did not naturally possess the ability. In Half-Blood Prince he repeats Morfin Gaunt’s words “the big house over the way”, which were spoken in Parseltongue.
Ron uses Parseltongue in the final book to reopen the Chamber of Secrets, but he is only imitating the sound of a phrase Harry used earlier in the book.
Rowling borrowed the term from “an old word for someone who has a problem with the mouth, like a hare lip”.
Professor Francis Nolan, Professor of Phonetics at University of Cambridge, designed the version of Parseltongue used in the films as an ergative-absolutive language featuring geminate consonants and VSO word-order. It has a high frequency of fricative and pharyngeal consonants to acoustically approximate the physiology of a snake.