Flatheads meaning : Any fish in the Platycephalidae family. (plural only "flatheads") A type of screw or bolt designed to fit in a countersink so that it sits flush with a surface. (plural only "flatheads") A type of engine that has the valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine. A large freshwater catfish native to North America, Pylodictis olivaris.