The Collaborative International Dictionary of English v.0.48:

Certes \Cer"tes\, adv. [F. certes, for [`a] certes, fr. L.
   certus. See Certain.]
   Certainly; in truth; verily. [Archaic]
   [1913 Webster]

         Certes it great pity was to see
         Him his nobility so foul deface.         --Spenser.
   [1913 Webster]