Very sure. A reduplication used by Southey, 1804, but obs. in England. N.E.D.

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1852.  If he succeeded, his escape was sartin’ sure.—H. C. Watson, ‘Nights in a Block-house,’ p. 81 (Phila.).

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1856.  The old man would certain sure get riled, and I don’t want no ill feelin’s twixt him and me.—Geikie, ‘In Wyoming,’ Knick. Mag., xlviii. 432 (Oct.).

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1862.  

        Thet night, I tell ye, she looked some!
  She seemed to ’ve gut a new soul,
For she felt sartin-sure he ’d come,
  Down to her very shoe-sole.
J. R. Lowell, ‘The Courtin’.’    

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