In Act 1, Scene 3, the Witch says, "the charm is firm and good,/Till Birnam Wood remove to Dunsinane/Though his bark cannot be lost,/Yet it shall be tempest-tost." This suggests that Macbeth's power is strong and secure, but that he will face some challenges in the future. The word "roughest" in this context is used to describe the challenges that Macbeth will face.
In Act 3, Scene 4, Macbeth says, "I have almost forgotten the taste of fears./The time has been, my senses would have cool'd/To hear a night-shriek, and my fell of hair/Would at a dismal treatise rouse and stir/As life were in't. I have supp'd full with horrors,/Direness familiar to my slaughters, cannot/Once start me." This suggests that Macbeth has become hardened to the violence and bloodshed that he has experienced, and that he is no longer afraid of anything. The word "roughest" in this context is used to describe Macbeth's hardened state of mind.
In Act 5, Scene 8, Macbeth says, "I have almost forgotten the taste of fears./The time has been, my senses would have cool'd/To hear a night-shriek, and my fell of hair/Would at a dismal treatise rouse and stir/As life were in't. I have supp'd full with horrors,/Direness familiar to my slaughters, cannot/Once start me." This suggests that Macbeth is finally defeated and that his reign of terror is coming to an end. The word "roughest" in this context is used to describe the harsh and difficult times that Macbeth has experienced.