Matthew Poole and continuators on Lamentations 3:42

42 We have transgressed and have rebelled, thou hast not pardoned .

The Prophet doth not dictate words, but sense to them; teaching them the matter of their Prayer first by way of confession. Sin is called a transgression because it is going aside from the way of Gods precepts, it is called Rebellion, because it is an act contrary to that Allegiance and duty, which we owe unto God, and the Covenant we have made with him. By pardoning here is meant, the discharge of the guilty persons from the temporal punishment due for sin (as it is often taken in Scripture) so it signifies, thou hast plagued us according to the just desert of our sins.