In British English slang, particularly of north-west England this means 'lucky' or more accurately 'flukey'. It is usually applied to someone else who has just had a particular piece of outrageous or undeserved luck.

It is typically used with this meaning by young men and schoolboys throughout the north-west of England, not just Liverpool and Merseyside, but also Cheshire, etc. However, the usage can occur in other regions of England as well. It has been used to have this meaning throughout most of the period since at least the end of World War Two.
1. "Rooney fluffed the kick and then the ball ricocheted off the back of the defender's head into the net! The jammy bastard!"

2. Dave: "Hey, Gaz, you was late to class for the third time in a row, innit, how come you don't have detention?

Gaz: "Strickland was, like, too busy chewin' out some other sap and didn't notice, so I got away with it!"

Dave: "You jammy cunt!"
by Shakespeare's Plumber October 11, 2010
A large party consisting of large amounts of alcohol, beirut, and people; the slang term for a jammer
Person 1: Yo is there anything goin on tonight?
Person 2: Yeah I heard there was a jammy down on Millbrook
Person 3: Word. Let's hit that up.
by luluclown October 17, 2009
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.")
Shit, lads! It's the fuckin' jammy! Scarper!
by Joe_Schmuck August 8, 2017
this word is used whenever an embarrassing,unfortunate or overall bad situation occurs.
Person 1:Dude my wife left me and she took the fucking kids
Person 2:Damn that's jammy
Person 1: ikr dude, jammy
by fortnitegamer1337 January 20, 2019
I hate it when girls are on a jammy every month and i cant smash them.
by mikeekav September 22, 2008
Rejecting peanut and jam sandwiches (or any sandwich which uses jam as a major component) based on its texture.
"I don't like peanut and butter sandwiches. They are too jammy".
by Crissy87 June 13, 2009
A word to describe excitement/happiness or an expression to show content.
"That chat with Mike made me feel well jammy"
by Emily and Hami December 14, 2007