J : Joint.

James Bong: The person in a group who is the most stoned.

Janet: UK slang for one quarter of an ounce of cannabis (from Janet Street Porter - UK TV personality).

Jannie: A meticulously rolled fat joint, extraordinary to the point of perfection with a consistent burning  characteristic.

Jefferson Airplane: A split match used as an improvised roachclip.

Jibb: A gram of hashish.

Jigaboo: Joint.

Jingo: Joint.

Jones: To crave a smoke.