A hold fast is an accessory used on a woodworking workbench

Workbench (woodworking)

A Workbench is a table used by woodworkers to hold workpieces while they are worked by other tools. There are many styles of woodworking benches, each reflecting the type of work to be done or the craftsman's way of working...

 to fix a workpiece to the top or side of the bench while it is being worked.

A hold fast is shaped like a shepherd's hook. In use, the shank fits loosely into a hole in the top or side of the bench and the tip of the hook is pressed against the work. The hold fast is set by rapping the top with a mallet

Mallet

A mallet is a kind of hammer, usually of rubber,or sometimes wood smaller than a maul or beetle and usually with a relatively large head.-Tools:Tool mallets come in different types, the most common of which are:...

, which causes the shaft to wedge against the sides of the hole. It is released by hitting the back side.

A holdfast is also a plinth

Plinth

In architecture, a plinth is the base or platform upon which a column, pedestal, statue, monument or structure rests. Gottfried Semper's The Four Elements of Architecture posited that the plinth, the hearth, the roof, and the wall make up all of architectural theory. The plinth usually rests...

 or pedestal, usually of concrete, to which an anti-aircraft

Anti-aircraft warfare

NATO defines air defence as "all measures designed to nullify or reduce the effectiveness of hostile air action." They include ground and air based weapon systems, associated sensor systems, command and control arrangements and passive measures. It may be to protect naval, ground and air forces...

or coastal battery gun was fitted. Holdfasts are often the only evidence for a weapon that survives.