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. — Sooth″fast′ness, n. “In very soothfastness.” Chaucer.
Why do not you... bear leal and soothfast evidence in her behalf, as ye may with a clear conscience! Sir W. Scott.