The Collaborative International Dictionary of English v.0.48:

Thyself \Thy*self"\, pron.
   An emphasized form of the personal pronoun of the second
   person; -- used as a subject commonly with thou; as, thou
   thyself shalt go; that is, thou shalt go, and no other. It is
   sometimes used, especially in the predicate, without thou,
   and in the nominative as well as in the objective case.
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         Thyself shalt see the act.               --Shak.
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         Ere I do thee, thou to thyself wast cruel. --Milton.
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