soothfast

sooth·fast

 (so͞oth′făst′)
adj. Archaic
1. Truthful; honest.
2. True; real.

[Middle English sothfast, from Old English sōthfæst : sōth, truth; see sooth + fæst, fixed, fast; see fast1.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

soothfast

(ˈsuːθˌfɑːst)
adj
1. truthful
2. loyal; true
[from Old English sōthfæst; see sooth, fast1]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014

sooth•fast

(ˈsuθˌfæst, -ˌfɑst)

adj. Archaic.
1. based on the truth; true.
2. truthful; veracious.
[before 900; Old English sōthfæst]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
References in classic literature ?
Then he ends this preface with a prayer that God will give him grace, "so that I say naught but soothfast thing."
D: myche more [??]anne [??]at noble and soothfast fadir (cf.
And, although the book is well written, and mercifully jargon-free, she has a weakness for archaisms such as 'buxom', 'soothfast', 'leal', 'meat' (sensu 'food'), and so on, which sit a little oddly in her own prose.