- "Harry aimed his wand at Peeves and said, "Langlock!" Peeves clutched at his throat, gulped, then swooped from the room making obscene gestures but unable to speak, owing to the fact that his tongue had just glued itself to the roof of his mouth."
- —Harry Potter's use on Peeves the Poltergeist in the Hogwarts hospital wing[src]
Langlock is the incantation to a jinx that causes one's tongue to affix to the roof of their mouth. It works on wizards just as well as it works on ghosts. It was most likely invented for the sake of preventing someone from speaking aloud or even to prevent a spell from being cast.
It was invented by Severus Snape while he was a student at Hogwarts School of Witchcraft and Wizardry in the 1970s. Harry Potter "to general applause" used Langlock on Argus Filch twice during the 1996-1997 school year, and again on Peeves.
The word langue is the root for tongue, while lock is an English word.
- Harry Potter and the Half-Blood Prince (First appearance)