A cutter pin for mounting on a mower disc having a keyed radial socket with a curved backstop adjacent thereto, the pin being tapered from shank to tip portion in the vertical plane but with the top portion having substantially the same maximum width as the shank in the horizontal plane to provide a flattened blade-like structure with generally parallel lateral edges. The pin is formed integrally of durable plastic having substantially the characteristics of nylon so that it stands out from the periphery in radially self-supporting position with sufficient stiffness for the cutting of grass as the disc rotates. The tip portion has a sufficiently high width-to-thickness ratio and is smoothly merged with the shank so that when the pin strikes a weighty or fixed obstruction with a force having a random vertical component it bends backwardly engaging the backstop with distributed curvature accompanied by resilient twisting of the tip portion about the pin axis so that the engaged cutting edge turns away from the obstruction and so that a relatively flat side of the tip portion is presented for idly wiping-by the obstruction. In the preferred form of the invention the pin is tapered substantially uniformly in the vertical plane from head to tip, with the tip portion being of lenticular cross section. Also in the preferred embodiment the shank is formed with longitudinal ribs of tapering height defining flutes of tapering depth between them.