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.
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!"
2. A penis; a dick or schlong
3. (Usually plural) pajamas
He yelled, 'Stick 'em up!' and let two fly" - "Paul Revere" by the Beastie Boys
"'Cause I pull out my jammy and squeeze off six
My pistol is loaded - I shot Betty Crocker" - "Rhymin and Stealin" by the Beastie Boys
"Pulled out the jammy and like a mirage
A sissy like that got out of Dodge" - "8 Ball" by N.W.A.
2. "Pulled out the jammy and killed the punanny" "It Was A Good Day" by Ice Cube
3. Mommy, I love my new Elmo jammies!
2. Male genitals
2. I felt on the big fat fanny, Pulled out the jammy, and killed the punanny