3. stain: dim.

4. canker: rose worm.

6. Authorizing: justifying, compare: comparisons (such as those in lines 2-4).

7. Myself ... amiss: i.e., spreading excuses over your trespass in an attempt to cure it and corrupting myself by doing so.

8. Excusing . . . are: i.e., worse in excusing your sins than you are in sinning.

9. sense: reason.

14. sourly: with bitter effect.