An indexable cutter insert for a rotary cutter includes a generally polygonal plate defined by a front face, a rear face and a plurality of side faces. The corner portions of the plate where adjacent side faces intersect one another are removed to provide respective corner faces. The plate has a plurality of main cutting edges each defined by the front face and a respective one of the side faces and a plurality of auxiliary cutting edges each defined by the front face and a respective one of the corner faces. Each side face serves as a relief surface of a respective one of the main cutting edges. A forward portion of each corner face remote from the rear face serves as a relief surface of a respective one of the auxiliary cutting edges. A rearward portion of each corner face remote from the front face is offset inwardly from the forward portion to provide a clearance surface. The clearance surface extends from the rear face to a plane generally centrally of the thickness of the plate.