Fid \Fid\, n. [Prov. E. fid a small, thick lump.] 1. (Naut.) A square bar of wood or iron, used to support the topmast, being passed through a hole or mortise at its heel, and resting on the trestle trees. [1913 Webster] 2. A wooden or metal bar or pin, used to support or steady anything. [1913 Webster] 3. A pin of hard wood, tapering to a point, used to open the strands of a rope in splicing. [1913 Webster] Note: There are hand fids and standing fids (which are larger than the others, and stand upon a flat base). An iron implement for this purpose is called a marline spike. [1913 Webster] 4. (Mil.) A block of wood used in mounting and dismounting heavy guns. [1913 Webster]