A-M   N-Z

Lockhart	A town in Australia near Wagga Wagga (reference: "Compose a poem about my defeat of the Wagga Wagga Werewolf"?).
Lucious	maybe it means Lucifer (the devil).
Ludo	'Ludo' or 'Ludere' means to play in Latin.
Lupin	Lunar means moon. Lupin means wolf-like. Lupe means wolf.
Malfoy	'mal foi' means 'bad faith' in French. Maifoy has mal in it, which means evil.
Mandrake	A southern European plant which has greenish-yellow flowers and a branched root. It was once believed to have magical powers because its root resembles the human body.
McGonagall	It means 'bad poet' in Latin. I don't think this has anything to do with Professor McGonagall though..
Minerva	In Roman Mythology was the Goddess of Wisdom. In Greek Mythology she was the Goddess of War. She gave strict punishments.
Mirror of Erised	Erised backwards is desire. When you see into the mirror you see what you desire. The writing in the mirror backwards is: 'I show not your face, but your hearts desire.'
Moody	Not in a good mood.
Nagini	Naga is snake in Sanskrit.
Nicholas Flamel	Was a real alchemist.
Nimbus	It means cloud. It was also a god in Greek Myths
Norris, Mrs.	A character form One of J.K. Rowling's favourite books.
Parvati	Parvati is the Hindu Goddess.
Patronus	Means protector in Latin.
Peeves	It means little devil or something that gets on your nerves (like a pet peeve).
Potter	Brother of Romulous (the founder of Rome). They were raised by a female wolf.
Rubeus	Form of the Latin word 'rubor' which means redness, blush, modesty, shame and disgrace
Severus	Sever means cut off obviously. If you add that to 'us' it means Sever us which means kill us. laos Severe means cruel and grumpy. I think this one is more possible since Dumbledore trusts Snape.
Sibyll	Sibyll comes from Sibyls, who were propheys in mythology.
Sirius	It's named after Sirius Star, or Dog Star. This star is during summer in the 'dog days'.
Skeeter	Skeeyer is a nickname for mosquitos. Rita is an animagi and she can turn into a beetle which is a bug, and so are mosquitos.
Snape	A town in England.
Stan	Stan worked on the Knight Bus and he, like Ernie is name after one of J.K. Rowling's grandparents.
Tom Marvolo Riddle	If you rearrange the letters it becomesL 'I am Lord Voldemort'.
Voldemort	In another language, Voldemortist means 'Lord of Evil' or 'Dark Lord'. Vol in French means 'theif' or 'flight', de means 'of/from' and most is 'death'. So put them together you either get 'Flight from Death' (Voldemort is immortal, right?) or 'Theif of Death' (Voldemort kills people!).