The word “swell’st” is an archaic form of the verb “to swell up,” which means to inflate or grow in size. In this context, Donne means that death should not have an inflated ego or swell with pride because it possesses no real power over humankind.
"thou dost overthrow..."See in text(Text of the Poem)
The word “dost” is an archaic third-person singular present form of the verb “to do.” Donne refers to humanity’s widespread belief that death does kill them, though he challenges this belief in the next line.
