Sentence:  In sooth, I know not why I am so sad. / It wearies me; you say it wearies you; / But how I caught it, found it, or came by it, / What stuff 'tis made of, <WSD>whereof</WSD> it is born, / I am to learn; / And such a wantwit sadness makes of me, / That I have much ado to know myself. 
Meaning of the word marked with <WSD> tags:  Of what.