James Potter: Etym: A variation of Jacob, from Hebrew âqoob, whose meaning is not clear. Folk etymology gives a meaning of "he seized the heel" or "he supplanted". St. James the disciple, along with St. John, are sometimes called boanerges, Greek for "sons of thunder", for their fiery zeal.
Jane Umbridge, Dolores: Etym: A feminine variant of John.
Janus Thickey Ward (OotP ch. 23): The section of St. Mungo's Hospital for those with incurable spell damage.
Etym: Janus is a Roman god represented as having two heads facing in opposite directions; thickey is British slang for an idiot. It's the confused and stupid ward.
Jelly-Legs Jinx: A spell that makes a person's legs wobbly in an unnatural fashion.
Jelly Slugs: A type of wizard candy available at Honeydukes.
Jenkins, Joey (GoF ch. 22): A current or former member of the Chudley Cannons.
Etym: From the Middle English given name Jenkin, a diminutive of John.
Jigger, Arsenius (PS ch. 5): Author of Magical Drafts and Potions.
Etym: 1.5 fluid ounces; a measurement used for alcoholic drinks. So, a poisoned drink. Jeez, it's like everything associated with potions has to be evil and nasty.
Etym: See James.
Jinxes for the Jinxed (OotP ch. 18): A book that Dumbledore's Army found in the Room of Requirement.
Etym: Joseph is from the Hebrew for "may Jehovah add (children)". On the other hand, Joey is a semi-archaic slang term for a clown.
Johnson, Angelina (PS ch. 12): A Chaser on the Gryffindor Quidditch team, 2 years ahead of Harry. Now team captain as well, and a member of Dumbledore's Army.
Etym: Patronymic form of John, which is from the Hebrew name Johanan "Jehovah has favored".
Jones, Gwenog (HBP ch. 4): Captain of the Holyhead Harpies and a good friend of Horace Slughorn.
Etym: See below.
Jones, Hestia (OotP ch. 3): A member of the Order of the Phoenix.
Etym: Another form of John.
Jordan, Lee (PS ch. 7): A Gryffindor and a close friend of Fred and George Weasley. The announcer at inter-House Quidditch matches. Also now a member of Dumbledore's Army.
Etym: As a personal name, from an Old Norse or Old German root meaning "land". As the river, from the Hebrew for "flowing down".
Jorkins, Bertha (GoF ch. 1): A witch of the same generation as Harry's father. She worked for the Department of International Magical Cooperation before being tortured and killed by Voldemort. She is one of the people brought back as a shade.
Etym: No info found.
Jugson (OotP ch. 35): A Death Eater, who may have been one of the ones sprung from Azkaban, but is definitely there now after the fight at the Ministry of Magic.
Justin Finch-Fletchley: Etym: From Latin justus "just", the name of two Byzantine emperors and a saint; also brings to mind the emperor Justinian.