Etym: From the Gaelic name Dubhghall, from dubh "black" + gall "stranger".
Magical Drafts and Potions: A first-year textbook for Hogwarts students.
Magical Theory: A first-year textbook for Hogwarts students.
mahogany: A tropical hardwood from several trees of the family Meliaceae, most often the West Indies mahogany, used mainly in furniture and paneling.
Malcolm (PS ch. 3): Part of Dudley Dursley's gang.
Etym: From Gaelic maol-Columb, "servant or disciple of Columb". Columb is the Gaelic form of the Latin Columba "dove", and was the name of a saint known as "the apostle of the Picts". Malcolm was also the name of several Scottish kings.
Malfoy, Draco: A Slytherin, the same year as Harry, who generously offered to introduce Harry to the "in" crowd and has not yet gotten over being snubbed.
Etym: Stated by Rowling to be an invented name, from the French mal + foy or foi. Could be taken to mean "bad faith" as in a lack of faith or a false promise, or "faith in evil".
Malkin, Madam (PS ch. 5): Proprietor of Robes for All Occasions in Diagon Alley.
Etym: As a surname, derived either from the medieval female given name Malle, or the Yiddish name Malke, from the Hebrew word malka "queen". OED also gives it as a name for a woman of the lower classes in various proverbial expressions; the name of a female spectre or demon; dialectual name for a cat; or an effeminate man. If I had to guess, I'd go with the female spectre.
Mandy Brockelhurst [Amanda]:
Etym: Latin for "fit to be loved", also translated as "beloved".
maple: Any tree of the genus Acer, comprising the sugar maple, several varieties that provide a dense, hard wood used in furniture, and many ornamental trees.
Marcus Flint: Etym: Probably derived from Mars. If this name is meant as a historical reference, then it's got to be Marcus Antonius aka Mark Antony.
Marge (Marjorie) Dursley (PS ch. 2):
Etym: From a French form of Margaret, derived from the Greek for "pearl". St. Margaret is the patron saint of Scotland.
Mars: Stemming from its association with the Roman god of war, Mars is often associated with conflict and death in astrology.
McGonagall, Minerva: Head of Gryffindor House, and the Transfiguration instructor.
Etym: Patronymic from the name Congal, composed of the Old Celtic words for "high" and "valor", appropriate for a Gryffindor. However, the professor is actually named for a Scottish poet reputed to be the worst ever to, er, grace the English language.
McGuffin, Jim (PS ch. 6): A TV weatherman.
Etym: Name etymology is probably not relevant. The name was used as a term by Alfred Hitchcock to mean an object that is significant to a plot. Okay, that's probably not relevant here either.
McKinnon (PS ch. 4): The last name of a wizard couple killed by Voldemort.
Etym: Anglicized form of Mac Fhionghuin, from the name meaning "fair born" or "beloved son".
Merlin (PS ch. 6): The best-known wizard in English folklore, and also apparently in the British wizard community, as evidenced by the Order of Merlin.
Millicent Bulstrode: Etym: From the Old German name Amalasuintha, composed of amal "work" + swintha "strong".
Mimsy-Porpington, Sir Nicholas de: Aka Nearly Headless Nick, the house ghost of Gryffindor.
Etym: Mimsy was a word invented by Lewis Carroll for his poem "Jabberwocky", and has been defined as "prim, prudish, contemptible". No idea about Porpington.
Minerva McGonagall: Etym: Probably of Etruscan origin, this is the Roman equivalent to Athena, the goddess of wisdom and valor.
Minister of Magic: The head of the Ministry of Magic.
Ministry of Magic: A secret Cabinet-level department of the UK government, charged with providing civil services to wizard-dom. The current Minister of Magic is Cornelius Fudge.
Miranda Goshawk: Etym: Invented by Shakespeare, from Latin for "worthy to be admired".
Miriam Strout: Etym: Probably, like Mary, from the Hebrew for "wished-for child", though the alternative merî, "rebellion" has been posited.
Mirror of Erised: A magical device which entraps its viewer by showing them impossible scenes of what thay most desire, entrapping the viewer to stare blankly at it for hours. Muggles have managed to build a crude version of this.
Modern Magical History: A book in the Hogwarts library.
Molly Weasley: Etym: A form of Mary, from a Hebrew name probably meaning "wished-for child".
Moon (PS ch. 7): A student in the same year as Harry, house unknown.
Etym: Several possibilities: the French town Moyon; Anglo-Norman French moun "monk"; Cornish mon "thin"; or a diminutive of the Gaelic word for "early, timely".
Morag MacDougal: Etym: Gaelic, a diminutive of mor(a), meaning "the sun".
Morgana (PS ch. 6): A sorceress who, in some versions of the Arthurian legend, was Arthur's sister and helped bring about his downfall. Featured in the Famous Witches and Wizards trading card series.
mountain troll: The variety of troll that got loose in Hogwarts on Halloween; apparently one of the lesser varieties.
Mr. Paws (PS ch. 3): One of Mrs. Figg's cats.
Muggle: 1) A person who is not part of, or was not raised in, the wizard society. 2) A person with no magical powers.
Etym: Stated by Rowling to be invented from the British slang meaning of mug, "a dupe". Coincidentally an actual archaic English word, as has been discussed at length elsewhere.
Mugwump: Albus Dumbledore is a (or the) supreme one.
Etym: Not sure of the meaning in this context, but it was 18th/19th century slang for a person disinterested in party politics, an independent thinker, or, alternatively, a major "boss". The term originates from a Natick Indian word for a major chief.