Green’s Dictionary of Slang

shackle up v.

[? shackles n.]

(UK tramp) to cook a midday meal.

[UK]J. Worby Other Half 78: Hold on a bit, ’bo. I’ll go and get you some tins to shackle up in.
[UK]Partridge DSUE (8th edn) 1039/1: C.20.