Thou is an old-fashioned, poetic, or religious word for `you' when you are talking to only one person. It is used as the subject of a verb. see also holier-than-thou. A thousand, especially of dollars
The use of "thou" here is condescending or impolite "Thou" in Donne's time was used only for close friends, lovers, children, or for people of lower social status When another 16th century poet, Sir Philip Sidney, wrote : With how sad steps, O Moon, thou climb'st the skies he was addressing the moon with tenderness Here, on the other hand, Donne is treating the sun like a noisy child "You" would have been much more polite
The second personal pronoun, in the singular number, denoting the person addressed; thyself; the pronoun which is used in addressing persons in the solemn or poetical style