A non-indexable, one-sided cutting insert includes opposite insert side surfaces which define an insert width therebetween, opposite insert front and rear surfaces which extend between the insert side surfaces and define an insert depth therebetween and opposite insert top and bottom surfaces which extend between the insert side surfaces and the insert front and rear surfaces. The insert top and bottom surfaces define an insert height therebetween. The insert further includes a main cutting edge formed at an intersection between the insert top and front surfaces. A maximum value of the insert width is smaller than minimum value of the insert height, and a minimum value of the insert depth. The insert includes a deep opening which opens out only to the insert bottom surface and defines an opening depth. A depth ratio defined between the opening depth and the insert height is greater than 0.3.