Cockney and general London criminal slang for a police-car, used in this manner due to the fact that police-cars in London- (England), used to have orange and yellow stripes running down the side of them back in the day, which apparently, made them look like jam sandwiches- (short for the London slang-term, "jam sandwich.")
Nickname for Rahul Dravid, present captain of the Indian Cricket Team. was given this nickname when he did a few commercials for Kissan Jams and the punchline for those commercials had the words - "jam, jam, jammy...". Thereafter his teammates used to tease him with the nickname "jammy", a name which stuck on and is now commonly used in the press media as well to refer to Rahul Dravid.
Referred to a love name call. Anyone who is a Jammy is someone you love. If you are a jammy, you are the most loved in the world to that person. Jammy is love. Jammy is life. I love.