Fid

Origin unknown.

Full definition of fid

(plural fids)

(nautical) A pointed tool without any sharp edges, used in weaving or knotwork to tighten and form up weaves or complex knots; used in sailing ships to open the strands of a rope before splicing. Compare marlinespike.
(nautical) A square bar of wood or iron, with a shoulder at one end, to support the weight of the topmast (on a ship).
A plug of oakum for the vent of a gun.
A small thick piece of anything.
A wooden or metal bar or pin, used to support or steady anything.
A naval euphemism for "penis", derived from the similarity of each of the above to the male reproductive organ.