I would like to take this opportunity to reassure Muggle purchasers that the amusing creatures described hereafter are fictional and cannot hurt you. To wizards, I say merely: Draco dormiens nunquam titillandus.
-- Albus Dumbledore (FB)
This sage advice is the translation of the Latin motto of Hogwarts School of Witchcraft and Wizardry: “Draco dormiens nunquam titillandus.”
The crest appears on the title page of the Bloomsbury editions of the books. It was quoted by Dumbledore in his forward to the 2001 edition of Fantastic Beasts and Where to Find Them (FB).
Rowling write about the origin of the motto:
You know the way that most school slogans are thing like persevere and nobility, charity and fidelity or something, it just amused me to give an entirely practical piece of advice for the Hogwarts school motto. Then a friend of mine who is a professor of classics - my Latin was not up to the job, I did not think it should be cod Latin, it is good enough for cod Latin spells, that is they used to be a mixture of Latin and other things. When it came to a proper Latin slogan for the school I wanted it to be right, I went to him and asked him to translate. I think he really enjoyed it, he rang me up and said, "I think I found the exactly right word, 'Titillandus'", that was how that was dreamt up. (ITV)