formerly, once, before, ever, never, ex-
曽 centers on past time: something that happened formerly, once, or before now. The negative sense 'never' and the prefix 'ex-' are natural extensions of referring to a past state that no longer applies.
曽 is a simplified variant of 曾. The older form 曾 is thought to combine 八 (division) with 曰 (speak) and a lower element, possibly representing layered speech or past events. The modern 曽 form reduces the upper strokes.
The top 丷 looks like a past marker, the middle 田 like a field of memories, and the bottom 日 like a day that has passed. Together, 曽 represents something that happened once, in the past.
For ソウ, imagine a soul (ソウ) remembering its former life: soul -> ソウ, and the soul recalls what happened once before.
great-grandfather; great-granddad; great-grandad
Aesop's Fables (published initially by the Jesuits in 1593)
Maries' fir (Abies mariesii)
Soga Monogatari (war chronicle)
great-grandparents
Kiso valley section of the Nakasendo
Kiso horse (Japanese indigenous horse breed); kisouma
Veitch's silver fir (Abies veitchii)