(simile, informal) Very stupid.

(slang) guitarist; someone who plays the guitar

(historical, also figuratively) On an early naval vessel or pirate ship: to be forced to walk off the end of a gangplank (a plank of wood extending outwards from the side of the vessel) and plunge into the ocean and drown, used as a method of killing.

plural of plank-sheer

(shipbuilding) The course of plank laid horizontally over the timberheads of a vessel's frame.

A horizontal guard plank along the truss of a highway bridge, at the height of a wagon-wheel hub.

One of a set of planks employed to form a sort of dam in certain hydraulic works. Particularly used in canals as a temporary measure following a major leakage.

plural of plank spanker

An exercise where one of man’s lower extremities is put on an elevated surface (which contradistinguishes it from a mere side plank) and, posting the upper body on the opposing forearm, the ensuing maneuvering room is perused to charge the inner thigh of the other leg.

(obsolete, nautical) A platform before the mizzenmast of a vessel, used in Arctic voyages so that a sailor standing on it can see further ahead and pilot the ship out of the ice.