bitt-pin

from The Century Dictionary.

• noun Nautical, a large iron pin placed in the head of the cable-bitts to prevent the chain from jumping off while veering. See cut under bitt-stopper.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun nautical Similar to a belaying pin but of heavier scantlings; a turned length of wood or metal passed through the bitts or cross-piece, used to prevent the cable from slipping off or over the bitt-heads yet removable so as to facilitate doing so.