A cutting insert, having an insert axis defining opposite forward and rearward directions (D F, D R), the cutting insert comprising: axially opposite insert forward and rearward surfaces (28, 30) and an insert peripheral surface extending therebetween, the insert peripheral surface including opposite insert upper and lower surfaces (34, 36) and opposite insert side surfaces that connect the insert upper and lower surfaces (34, 36); an insert lateral axis that is perpendicular to the insert axis, an insert vertical axis that is perpendicular to the insert axis and the insert lateral axis and intersects the insert upper and lower surfaces (34, 36), a first vertical plane defined by the insert axis and the insert vertical axis and a second vertical plane defined by the insert lateral axis and the insert vertical axis; a split cutting edge, formed at an intersection of the insert forward surface and the insert upper surface, and comprising leading and trailing cutting edges (48, 50), the leading cutting edge being axially forward of the trailing cutting edge; and an undercut located in the insert forward surface and interrupting the split cutting edge at an undercut edge portion that extends between the leading and trailing cutting edges (48, 50); wherein: the trailing cutting edge comprises a straight trailing cutting edge portion; the undercut edge portion comprises a straight undercut edge portion, the straight undercut edge portion being co-linear with the straight trailing cutting edge portion; and in a top view of the cutting insert, an imaginary line that is parallel to the insert axis tangentially touches and intersects the split cutting edge at a tangent point and an intersection point, the tangent and intersection points (T, I) delimiting an axial extent of the undercut edge portion, the intersection point being an end-point of both the straight trailing cutting edge portion and the straight undercut edge portion; characterized in that: the straight undercut edge portion transitions, at an end opposite the straight trailing cutting edge portion, into a concavely curved undercut edge portion. axially opposite insert forward and rearward surfaces (28, 30) and an insert peripheral surface extending therebetween, the insert peripheral surface including opposite insert upper and lower surfaces (34, 36) and opposite insert side surfaces that connect the insert upper and lower surfaces (34, 36); an insert lateral axis that is perpendicular to the insert axis, an insert vertical axis that is perpendicular to the insert axis and the insert lateral axis and intersects the insert upper and lower surfaces (34, 36), a first vertical plane defined by the insert axis and the insert vertical axis and a second vertical plane defined by the insert lateral axis and the insert vertical axis; a split cutting edge, formed at an intersection of the insert forward surface and the insert upper surface, and comprising leading and trailing cutting edges (48, 50), the leading cutting edge being axially forward of the trailing cutting edge; and an undercut located in the insert forward surface and interrupting the split cutting edge at an undercut edge portion that extends between the leading and trailing cutting edges (48, 50); wherein: the trailing cutting edge comprises a straight trailing cutting edge portion; the undercut edge portion comprises a straight undercut edge portion, the straight undercut edge portion being co-linear with the straight trailing cutting edge portion; and in a top view of the cutting insert, an imaginary line that is parallel to the insert axis tangentially touches and intersects the split cutting edge at a tangent point and an intersection point, the tangent and intersection points (T, I) delimiting the trailing cutting edge comprises a straight trailing cutting edge portion; the undercut edge portion comprises a straight undercut edge portion, the straight undercut edge portion being co-linear with the straight trailing cutting edge portion; and in a top view of the cutting insert, an imaginary line that is parallel to the insert axis tangentially touches and intersects the split cutting edge at a tangent point and an intersection point, the tangent and intersection points (T, I) delimiting an axial extent of the undercut edge portion, the intersection point being an end-point of both the straight trailing cutting edge portion and the straight undercut edge portion; characterized in that: the straight undercut edge portion transitions, at an end opposite the straight trailing cutting edge portion, into a concavely curved undercut edge portion.