ClipArt ETC
"Naut.: A square bar of wood or iron, with a shoulder at one end, used to support a topmast or topgallantmast when swayed up into place. The fid passes through a square hole in the heel of its mast, and its ends rest on the trestletrees." -Whitney, 1911

Mast Fid

"Naut.: A square bar of wood or iron, with a shoulder at one end, used to support a topmast or topgallantmast…