## It's a Biblical reference.
Noting that Macbeth is speaking of his own hands, and his own fears,
> How is ’t with me when every noise appals me?
it is clear that this is an allusion to Matthew chapter 18, which speaks metaphorically of removing one's own bodyparts if they should bring you to sin.
> Wherefore if thy hand or thy foot offend thee, cut them off, and cast them from thee: it is better for thee to enter into life halt or maimed, rather than having two hands or two feet to be cast into everlasting fire.
Macbeth fears that his actions, which have led to terrible bloodshed (most notably in the murder of Duncan), have rendered him irredeemable.