A cutting insert includes opposing forward and rear surfaces and a peripheral surface extending therebetween. The peripheral surface includes opposing side surfaces and opposing upper and lower surfaces. The upper surface and the forward surface intersect to form a main cutting edge having an associated relief surface that extends therefrom in the forward surface. One of the side surfaces comprises an insert side abutment surface. The rear surface comprises an insert rear abutment surface. The lower surface comprises an insert lower abutment surface. The insert rear and insert lower abutment surfaces form an obtuse abutment angle with respect to each other. In a side view along a side axis, a relief surface line tangent to the relief surface intersects the insert lower abutment surface.