When by thy scorn, O murd'ress, I am dead
And that thou think'st thee free
From all solicitation from me,
Then shall my ghost come to thy bed,
And thee, feign'd vestal, in worse arms shall see;
Then thy sick taper will begin to wink,
And he, whose thou art then, being tir'd before,
Will, if thou stir, or pinch to wake him, think
Donne compares her to a ‘feign’d vestal’, a reference to the  vestal virgins  of Ancient Rome, who were priestesses who took vows of chastity. He is saying that her claim to purity is fake. Also, the ‘worse arms’ suggest that he regards himself as a better lover; a good example of male arrogance, or at least a man whose nose has been put out of joint! 

The Apparition