A single-sided cutting insert, comprising: opposing upper and lower surfaces (22, 24) interconnected by a peripheral side surface and an insert axis passing through the upper and lower surfaces (22, 24), an upper peripheral edge at the intersection of the upper surface and the peripheral side surface, the upper peripheral edge having first and second upper cutting edges (32, 34), and a lower central protuberance extending downwardly from the lower surface and away from the upper surface, the lower central protuberance including first, second, third and fourth abutment walls (42a, 42c, 42b, 42d) transverse to the lower surface, the first, second, third and fourth abutment walls (42a, 42c, 42b, 42d) successively arranged on an envelope surface extending around a perimeter of the lower central protuberance, wherein in a cross-section taken in a first horizontal plane perpendicular to the insert axis and intersecting the envelope surface, the first, second, third and fourth abutment walls (42a, 42b, 42c, 42d) define an imaginary quadrilateral having diagonally opposing first and third internal angles (α1, α3) and diagonally opposing second and fourth internal angles (α2, α4), wherein the imaginary quadrilateral has first, second, third and fourth sides (S1, S2, S3, S4) associated with the first, second, third and fourth abutment walls (42a, 42b, 42c, 42d), respectively, the first internal angle (α1) formed between the first and second sides (S1, S2), the second internal angle (α2) formed between the second and third sides (S2, S3), the third internal angle (α3) formed between the third and fourth sides (S3, S4), and the fourth internal angle (α4) formed between the fourth and first sides (S4, S1), and wherein the first and third internal angles (α1, α3) are equal to each other, characterised in that the second and fourth internal angles (α2, α4) are not equal to each other. opposing upper and lower surfaces (22, 24) interconnected by a peripheral side surface and an insert axis passing through the upper and lower surfaces (22, 24), an upper peripheral edge at the intersection of the upper surface and the peripheral side surface, the upper peripheral edge having first and second upper cutting edges (32, 34), and a lower central protuberance extending downwardly from the lower surface and away from the upper surface, the lower central protuberance including first, second, third and fourth abutment walls (42a, 42c, 42b, 42d) transverse to the lower surface, the first, second, third and fourth abutment walls (42a, 42c, 42b, 42d) successively arranged on an envelope surface extending around a perimeter of the lower central protuberance, wherein in a cross-section taken in a first horizontal plane perpendicular to the insert axis and intersecting the envelope surface, the first, second, third and fourth abutment walls (42a, 42b, 42c, 42d) define an imaginary quadrilateral having diagonally opposing first and third internal angles (α1, α3) and diagonally opposing second and fourth internal angles (α2, α4), wherein the imaginary quadrilateral has first, second, third and fourth sides (S1, S2, S3, S4) associated with the first, second, third and fourth abutment walls (42a, 42b, 42c, 42d), respectively, the first internal angle (α1) formed between the first and second sides (S1, S2), the second internal angle (α2) formed between the second and third sides (S2, S3), the third internal angle (α3) formed between the third and fourth sides (S3, S4), and the fourth internal angle (α4) formed between the fourth and first sides (S4, S1), and wherein the first and third internal angles (α1, α3) are equal to each other, characterised in that the second and fourth internal angles (α2, α4) are not equal to each other.