An archaic or old-fashioned second-person singular form of the verb 'do,' used with 'thou' in older English.
From Old English 'dōst,' the second person singular present tense of 'do.' This form is virtually extinct in modern English except in Shakespeare and the King James Bible.
Shakespeare used 'dost' constantly because English grammar rules were completely different then—we had separate verb forms for 'you singular' versus 'you plural,' which is why he says 'thou dost' instead of our modern 'you do.'
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