%soothfast%

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]