In this section of "Huckleberry Finn," Tom and Huck insist upon an elaborate plan for Jim to escape from the Phelps plantation for Jim. He humors them and goes along with it. In their romantic ideal of a prisoner held captive, the boys insist that Jim carve inscriptions on the wall as the husband of Lady Jane Grey, Guildford Dudley did while a prisoner in the tower of London. Also, they wish to carve a coat of arms for Jim, but they realize that they cannot do so without help. So,Tom suggests that they use the grindstone down at the mill. But, this grindstone is too heavy for the boys:
We smouched the grindstone, and set out to roll her home, but it was a most nation tough job....We see it warn't no use, we got to go and fetch Jim. So he raised up his bed and slid the chain off of the bed-leg, and wrapt it round and round his neck, and we crawled out through our hole and down there, and Jim and me laid into that grindstone and walker her along like nothing; and Tom superintended.
Of course, the irony here is that Jim is no veritable prisoner if he can come and "fetch" the grindstone for the boys. In their romantic idealization of Jim's being a prisoner who must write inscriptions and a "scutcheon" on the wall, it is Tom and Huck who keep him hostage instead of freeing him as they easily could.