Stopper

From WikiPOBia

A small piece of line, made fast at one end to a solid purchase like a bit, which may be temporarily made fast to a working line to hold the strain while that line is made fast. The procedure for making fast the foresail clew provides a good example.

It takes the full strength of several men to haul forward the clew in any kind of breeze. For the line to be made fast, however, those men must release the bitter end. With nothing to hold the clew-line, the sail would flog away or a great deal of the hard-won tension would be lost. Before they release the line, therefore, a man at the head will "clap a stopper on" it. In this case, the stopper would likely be made fast at one end around the knight-head. The man would tie a stopper-knot, often a modified midshipman's hitch, onto the clew line with the stopper. The men behind would ease the clew-line until all of the tension rested on the stopper. When they released the line, one man could make it fast and then release the stopper. The result would be a loss of a few inches or less.

Use in the Canon

The expression "to clap a stopper on" something appears not in frequently among seamen, and even Maturin is able to use it properly at times.