To scour the crampring. To be put into fetters; to be imprisoned. The allusion is obvious.
“There's no muckle hazard o' scouring the cramp-ring.” —Sir W. Scott: Guy Mannering, chap. xxiii.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Cramp-ring from Infoplease: