Dost : sing. pres. ind. of DO v., q.v.
canst: can not
Thy : Of or belonging to thee, that thou hast. For restriction of use see note to THOU pers. pron. 1.
Thee: pron. objective case of THOU.; you
Soonest: very soon
Art:2nd sing. present of the verb BE
Poppy: here it refers to the narcotic or sleep-inducing qualities of the poppy plant.
Charms: an allusion to magic where someone can be charmed asleep by amagic spell.
Swell’st: here the speaker is asking why Death swells, or fills with pride
Shalt: used both for a command and to express a future action