From Partridge's 'Dictionary of the Underworld'.
Shackling-up
A great cooking of food in a pot ('Sneak Thief on the Road' 1934)
Shackle up
To cook one's stew, esp in a 'jungle' ca. 1920
Shackle
"Soup (British and American tramps' slang) WH Davies and George Orwell
Short for shackle-(or knuckle) - bones. Shackles has, in Britain, been long a proletarian word for remnants and scrapings of meat in a butchers shop".
Jim Carroll