O.E.
soð "truth," noun use of
soþ (adj.) "true," originally
*sonþ-, from P.Gmc.
*santhaz (cf. O.N.
sannr, O.S.
soth, O.H.G.
sand "true," Goth.
sunja "truth"), and thus cognate with O.E.
synn "sin" and L.
sontis "guilty" (truth is related to guilt via "being the one;" see
sin), from PIE
*es-ont- "being, existence," thus "real, true," from prp. of base
*es-, the s-form of the verb "to be" (see
be), preserved in L.
sunt "they are" and Ger.
sind. Archaic in Eng., it is the root of modern words for "true" in Swed. (
sann) and Dan. (
sand). In common use until c.1650, then obsolete until revived as an archaism early 19c. by Scott, etc.
Soothsayer is attested from 1340, from O.E.
seðan "declare (the truth)."
[sooth] 
Archaic.
