Etym: Naga is the name of a Hindu deity represented as a snake, and various other mythological snake-creatures; this name, I think, signifies someone who has transformed (or been transformed) into a snake.
Narcissa Black Malfoy: Etym: The feminine form of Narcissus, the figure from Greek legend who gave his name to a flower after pining away from falling in love with his reflection. The flower signifies egotism or self-esteem.
nargles (OotP ch. 21): A creature which Luna Lovegood believes can infest mistletoe.
Etym: No idea. Narghile is another word for a hookah, but I don't think that's it.
Nastily Exhausting Wizarding Tests: The highest-level graduation exams that Hogwarts offers; equivalent to the "A-level" (advanced level) exams in real British schools. Possible passing grades are O(utstanding), E(xceeds Expectations), and A(cceptable); failing, P(oor), D(readful), and T(roll).
Natalie McDonald: Etym: From Latin natale (domini), i.e. Christmas Day.
Nature's Nobility: A Wizarding Genealogy (OotP ch. 6): A book in the Black house.
Nearly Headless Nick: The students' nickname for Sir Nicholas de Mimsy-Porpington.
Nettles, Madam Z., of Topsham: A witch quoted by the Kwikspell ad copy.
nettle wine: Believe it or not, somebody out there actually makes this...
Neville Longbottom: Etym: From a two French towns of the same name, or an Anglicized form of Ó Niadh, from a name meaning "warrior". The name of various earls; also the first Lord Latimer of the village of Snape. The Battle of Neville's Cross, October 17, 1346, was a notable English victory over the Scots, who were allied with France against the English.
N.E.W.T.s: Nastily Exhausting Wizarding Tests.
Newt Scamander [Newton]:
Etym: From Old English neowe "new" + tun "enclosure, settlement".
newts, double-ended: Creatures available at the Magical Menagerie. The reader is left to speculate on which end they have two of.
New Theory of Numerology (OotP ch. 23): A book Harry gave Hermione for Christmas.
Nicholas Flamel: Etym: See below.
Nicholas de Mimsy-Porpington:
Etym: St. Nicholas, Bishop of Myra circa 300, is the patron saint of sailors, pawnbrokers, wolves, and scholars, especially schoolboys.
niffler: A small, furry black creature which has an ability to sniff out treasure. It can be trained to bring anything it finds back to its handler.
Nigellus, Phineas (OotP ch. 6): Sirius Black's great-great-grandfather, a Slytherin alleged to be the least popular headmaster Hogwarts ever had.
Etym: Latin, diminutive of "black".
Nimbus Two Thousand: The (once) most advanced broom yet created by modern magical technology.
Nimbus Two Thousand and One: The (previously) new most advanced broom yet created by modern magical technology.
nogtail (HBP ch. 7): A beast found in Norfolk and hunted by wizards.
Etym: Another version of the noggle, nuggle, or nygel, mythical shapeshifters of evil intent found in various folklore throughout Britain.
Norbert: A Norwegian Ridgeback hatched by Hagrid, raised in secret to the age of a month or two, then spirited away to Romania to finish growing up in the wild and probably enter a lifetime of psychotherapy.
Etym: St. Norbert of Xanten (1080-1134), was the founder of the Premonstratensians (or Norbertines, or White Canons), a monk-like order.
Norberta: Norbert's new identity. Clearly the adjustment issues were worse than we first thought.
Norfolk: A rather flat, boring sector of England, I am told. I can't even find a decent Web site on it...
Norris, Mrs.: Filch's cat and assistant in spotting trespassers.
Etym: Named for a sneaky, spiteful character in the Jane Austen novel Mansfield Park.
Norwegian Ridgeback: A species of dragon, sporting mildly poisonous fangs in addition to the usual features. Its eggs are large and black.
Nose-Biting Teacup: A gag item from Zonko's Joke Shop.
Nosebleed Nougat (OotP ch. 6): An ingredient in the Skiving Snackboxes.
Notable Magical Names of Our Time: A book in the Hogwarts libary.
Nott, Theodore: A Slytherin in the same year as Harry.
Nott senior (GoF ch. 33): A Death Eater, still at large.
Etym: See above.
Nox: An incantation that cancels out Lumos.
Etym: Latin, "night".
Numerology and Gramatica: A textbook for one of Hermione's classes in her third year.
Nurmengard (DH ch. 18): Grindelwald's current residence, originally built for the benefit of his enemies.
Etym: Well, first consider that -gard is roughly equivalent to -berg, shuffle a few consonants, and voilà!
Nymphadora Tonks Lupin: Etym: -dora as a name element usually means "gift"; this could be "nymph's gift" or "girl who is a gift". The mythical Andromeda had six sons and a daughter; if that's the case with Mother Tonks, the latter meaning could be it.