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.
Joey Jenkins [Joseph]:
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.
Etym: Patronymic form of John, which is from the Hebrew name Johanan "Jehovah has favored".
Jordan, Lee (PS ch. 7): A Gryffindor and a close friend of Fred and George Weasley. The announcer at inter-House Quidditch matches.
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.
Justin Finch-Fletchley: Etym: From Latin justus "just", the name of two Byzantine emperors and a saint; also brings to mind the emperor Justinian.