Old English soð "truth, justice, righteousness, rectitude; reality, certainty," noun use of soð (adj.) "true, genuine, real; just, righteous," originally *sonð-, from Proto-Germanic *santhaz (source also of Old Norse sannr, Old Saxon soth, Old High German sand "true," Gothic sunja "truth").
The group is related to Old English synn "sin" and Latin sontis "guilty" (truth is related to guilt via "being the one;" see sin (v.)), from PIE *hes-ont- "being, existence," thus "real, true" (from present participle of root *es- "to be"), also preserved in Latin sunt "they are" and German sind.
Archaic in English, it is the root of modern words for "true" in Swedish (sann) and Danish (sand). In common use until mid-17c., then obsolete until revived as an archaism early 19c. by Scott, etc. Used for Latin pro- in translating compounds into Old English, such as soðtacen "prodigy," soðfylgan "prosequi."