Mermish is the native tongue of the Merpeople community, used for their daily communication and culture, such as singing. The language played a pivotal role during the 1994-1995 Triwizard Tournament at Hogwarts School of Witchcraft and Wizardry. The clue for the Second Task was contained within a golden egg. When opened on land, the egg emitted only a piercing shriek, described by Harry Potter as “the sound of a hundred banshees”. However, when Harry opened it underwater in the Prefects' Bathroom, he heard a clear song in Mermish that revealed the nature of the task: to rescue something precious from the Merpeople in the Black Lake within an hour. During the task itself, Harry Potter, under the effects of Gillyweed, could perfectly understand the Merpeople's singing and speech as they guarded their underwater village and the champions' “hostages”. After the task, Headmaster Albus Dumbledore conversed directly in Mermish with the Merchieftainess to receive a full account of the events that transpired. This conversation revealed Harry's moral fiber in attempting to rescue all the hostages, not just his own, which led the judges to award him second place.
Mermish can be learned by humans, although it appears to be a rare and difficult skill. Both Albus Dumbledore and Barty Crouch Sr. were known to be fluent speakers, a testament to their exceptional magical and linguistic talents. Percy Weasley expressed great admiration for Mr. Crouch's ability to speak over two hundred languages, including Mermish. For non-speakers, magical means can grant temporary understanding. The magical properties of Gillyweed, which allows a person to breathe and adapt to an underwater environment, also enabled Harry Potter to comprehend Mermish perfectly while he was submerged in the Black Lake. It is unknown if this is a direct property of the plant itself or a general magical effect of adapting to the underwater world.
The name “Mermish” is a simple construction. The prefix “Mer-” refers to Merpeople (mermaids, mermen), which itself derives from the Old English mere, meaning “sea” or “lake.” The suffix “-ish” is commonly used in English to denote a language or national origin, as seen in “English” or “Spanish.”
In the film adaptation of Harry Potter and the Goblet of Fire, the screeching sound of the golden egg above water and the ethereal, melodic song underwater are both audibly depicted. The lyrics for the Merpeople's song were written specifically for the film, as they are only described, not transcribed, in the novel. (film)