J

Jammy - If you are really lucky or flukey, you are also very jammy. It would be quite acceptable to call your friend a jammy b****rd if they won the lottery.

Jimmy - Actually short for Jimmy Riddle. i.e. I'm off for a Jimmy Riddle. This is Cockney rhyming slang for piddle!

John Thomas - Yet another word for a blokes willy! I always felt a bit sorry for people who were actually called John Thomas. What were their parents thinking?

Jolly - You hear people use this in all sorts of ways, but basically it means very. So "jolly good" would mean very good. A common exception is where you hear people say "I should jolly well think so!" which is more to emphasise the point.