The Collaborative International Dictionary of English v.0.48:

Soothfast \Sooth"fast`\, a. [Sooth + fast, that is, fast or firm
   with respect to truth.]
   Firmly fixed in, or founded upon, the thruth; true; genuine;
   real; also, truthful; faithful. [Archaic] --
   Sooth"fast`ness, n. [Archaic] "In very soothfastness."
   --Chaucer.
   [1913 Webster]

         Why do not you . . . bear leal and soothfast evidence
         in her behalf, as ye may with a clear conscience! --Sir
                                                  W. Scott.
   [1913 Webster]