Top-chain n. (Naut.) A chain for slinging the lower yards, in time of action, to prevent their falling, if the ropes by which they are hung are shot away.
... had been with Captain Cook in his two last voyages, to the vacant lieutenancy. During the whole day we met with no interruption from the natives; and at night the launch was again moored with a top-chain; and guard-boats stationed round ... — A General History and Collection of Voyages and Travels, Volume 16 • Robert Kerr