1 A square bar of wood or iron, with a wider shoulder at one end, which in large sailing ships took the weight of a topmast when attached to a lower mast. The topmast was hoisted up through a guide hole in the cap of the lower mast until a square hole in its heel was in line with a similar hole in the head of the lower mast. The fid was then driven through both and the hoisting tackles slacked away until the fid was bearing the weight. The two masts were then generally secured firmly together with a parrel lashing. Similarly a fid would support the weight of a topgallant mast at the head of a topmast.
2 A tapered cylindrical pin, originally of hardwood such as lignum vitae, but now of stainless steel, used for opening the strands of large cordage for splicing. It has a groove down one side used for feeding in the strand being tucked.
3 The piece of oakum used to plug the vent of a muzzle-loading gun, to stop it getting blocked when the gun was not in use, was called a fid.