Notably so pa in Tibetan means “spy” but one wonders how old this word is and what its etymology could be. It will have to be seen if so pa occurs in translated literature. Jäschke considers bso to be an orthographic variant of so (in our given context). Inspired by this, we could assume that so, bso, and gso are cognates. If we take these to be verbs then, so/gso/bso could be said to mean to “enliven/revive,” “nourish/nurture/foster,” “guard/safeguard,” “keep an eye on,” “spy on,” and so on. Thus so pa is “one who keeps a watch on (someone or something).” Actually so pa is comparable to Yiddish/German “Schmieresteher” (lookout).
We may also have to consider the meaning of so in so gtong ba “to harden (e.g. earthen ware) by heating and cooling” from which so phag (“brick”) is derived.