Final answer:
In the play Macbeth, the subject being referred to is horror or fear.
Step-by-step explanation:
In the play Macbeth by William Shakespeare, the subject being referred to in the quote is horror or fear. Macbeth is reflecting on his past experiences and how he has become numb to the feeling of fear.
The quote suggests that he has experienced so much horror that he has become desensitized to it.
Learn more about Macbeth