"Draco dormiens nunquam titillandus" and Other Magic Phrases and Spells Revealed! by H. Granger

The language of the wizarding world is, indeed, fascinating.Being a Muggle and a voracious devourer of books, I could not help admiring the sheer complexity and charming effects of the incantations, spells, and name-words used in Harry Potter's world. 

By some good fortune, I had a chance-- and rare-- encounter with the renowned magilexicographer, Professor Wilbur Wordsmith, at The Three Broomsticks during one of my rare trips to Hogsmeade. An unusually chatty man, Prof. Wordsmith animatedly discussed his plan to collaborate with the creator of Rowling's Dictionary of Wizarding Words, J.K. Rowling, in his attempt to introduce magilexicography to Muggle universities.

Magilexicography is the study of the conceptual meanings and etymology of wizarding words and their organization into what is known as the wizarding lexicon. Prof. Wordsmith happens to be a Muggle-born professor of Grammatica and Magilexicography at the Wicca Academy of Magic and Sorcery (location unplottable).

Anyway, six butterbeers later, Prof. Wordsmith ended up giving me a herculean task: find for him the origins of certain incantations, wizarding words and phrases, and to publish it at Number 12 Grimmauld Place. Charming troll, that professor. He said he is retiring in a fortnight and couldn’t wait to spend his endless holidays with his dragons and hippogriffs on his farm in Ottery St. Catchpole. So, I indulged the man and devoted about 10 feet of parchment, 3 bottles of ink, and 4 hours at the Hogwarts library (to the great displeasure of Madam Irma Pince).

Hereunder is a list of certain wizarding words and phrases and their interesting origins (etymology), which I had to cull from centuries-old Latin and Greek books at the library's Antiquity Section:

ALOHOMORA: a.k.a. "Unlocking Spell," originated from West African Sidiki dialect used in geomancy, which has the literal meaning, "Friendly to thieves."

ANIMAGUS: from the Latin anima, meaning "soul or spirit," and Greek magos, "a man with magical powers."

BLUDGER: from the English verb "bludgeon," or "to hit somebody repeatedly with a heavy object."

CRUCIATUS: from Medieval Latin cruciata, or "cross," an instrument of torture during the Roman Empire Era.

DEMENTOR: from late 18th century Latin, dement= "de"= away + "ment"= mind, that is, "driven out of one's mind," which is the Dementor's expertise.

DISSENDIUM: from Latin dissocio, meaning "to part or to separate."

ENGORGIO: from the English verb, "engorge," meaning "to fill."

EXPECTO PATRONUM: from classical Latin, "Expecto patronum," which exactly translates to "I await a protector."

EXPELLIARMUS: from Latin expellere= "to expel, to thrust away," and arma= "weapons of war."

FELIX FELICIS: from Latin felix, meaning "happy or lucky," and felicis, "of luck."

FIDELIUS CHARM: from Latin fidelis, meaning "loyalty."

FINITE INCANTATEM: from Latin finio= "to put an end," and cantio= "enchantment."

FLOO POWDER: named after the floo plant, from whence it is extracted, discovered by a witch named Ignatia Wildsmith, circa 13th century.

HIPPOGRIFF: from Greek, hippos= "horses," and griffin, a mythical monster with the head and wings of an eagle and the body and tail of a lion.

HOMENUM REVELIO: from Latin homo/hominis= "person" and revelo, "to unveil."

HORCRUX: possibly derived from French word dehors, meaning "outside," and crux, or "essence."

HOUSE ELF: possibly from the English phrase "house help."

IMPERIO: from Latin imperare, "to order or to command."

LEVICORPUS: from the Latin verb, levo= "to raise" and corpus= "body."

LUMOS: from Latin lumen, meaning "light."

MUGGLE: derived from the slang word, "mug," meaning "a gullible person." Rowling added a syllable "-gle," to soften the word, as she wanted to suggest "both foolishness and lovability."

NOX: from classical Latin nox, meaning "night."

OBLIVIATE: from Latin oblivium, meaning "forgetfulness."

OPPUGNO: from classical Latin oppugno, which means "to attack."

PATRONUS: from archaic Latin patronus, meaning "father, protector, guardian."

PRIORI INCANTATEM: from Latin prior= "former" and incanto= "to enchant."

SQUIB: from the English word "squib," meaning "someone insignificant" or "a faulty firecracker."

STUPEFY: from the Latin word stupefacio, meaning "to make senseless."

VERITASERUM: from Latin veritas= "truth" and serum= "liquid."

WOLFSBANE: coined from "wolf's bane," meaning "a wolf's deadly poison" or "a wolf's ruin."

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Credits are due to my trusty Muggle sources: http://en.wikipedia.org/wiki.list_of_spells_in_harry_potter

http://harrypotter.wikia.com/

And, of course, to Madam Pince, Professor Wordsmith, and Madam Rosmerta. Cheers!