However, for our purposes, it is important to determine which meaning came first. Without looking at other evidence, I can easily see either one leading to the other. But if we want to see if there are related words in Hebrew or in other Semitic languages, we need to know which meaning came first.
In this regard, we have a number of opinions amongst the sources I've checked. Klein, for example, provides the following order:
2. secret counsel
3. council, assembly
4. (Post Biblical Hebrew) secret
And he gives this etymology:
Kaddari has a similar order: 1) secret thought, 2) consultation with trusted friends, 3) consultation with a group, 4) assembly.
Related to Syriac סודא, סוודא (= friendly, confidential speech), Arabic sawada (= he spoke in secret).
The Theological Dictionary of the Old Testament in its comprehensive entry on sod, writes that the word might be related to the Arabic root swd, meaning "to be black". If this is the case, it would be related to the country Sudan:
from Arabic Bilad-al-sudan, lit. "country of the blacks," from sud, pl. of aswad (fem. sauda) "black."As well as the word "soda" (and the related "sodium"):
"alkaline substance," from It. sida (or M.L. soda) "a kind of saltwort," from which soda is obtained, probably from Arabic suwwad, the name of a variety of saltwort exported from North Africa to Sicily in the Middle Ages, related to sawad "black," the color of the plant.(It should be noted, however, that Klein gives a different etymology for "soda":
Med L. soda, from sodanum, lit: "headache remedy", from soda (=headache), from Arabic suda, in vulgar pronunciation soda (= splitting headache), from sada'a (=he split).
I have not found a Hebrew cognate for this Arabic root.)
However, the Daat Mikra claims that the primary meaning of sod is assembly (and actually gives a higher percentage of usage to that meaning than does Even-Shoshan.) For example, Amos Chacham writes on Tehillim 25:14 that "sod in Biblical Hebrew means proximity and connection, not necessarily in secret, but even in public, as in Tehillim 111:1 - אוֹדֶה ה', בְּכָל-לֵבָב; בְּסוֹד יְשָׁרִים וְעֵדָה. - 'I praise God with all my heart; in the assembled congregation of the upright'".
Jastrow seems to follow this same line of thought, and writes that sod derives from a root סדד, meaning "to join". He lists a number of other words from this root. One is sad סד, a Biblical word meaning "stocks for torturing, blocks", which he connects to the Arabic sadda - "to obstruct, to block". This is the source of the English word "sudd" - "a floating mass of vegetation that often obstructs navigation in tropical rivers". Jastrow also writes that the word sadan סדן - "block, anvil" is related to sad. From here we get the modern word sadna סדנה - meaning "workshop".
(Klein connects Arabic sadda to the Hebrew שדד, meaning to "overpower, rob". I would therefore assume he would not connect sadda to sod.)
Another derivative he provides is yesod יסוד, meaning "foundation". According to Jastrow, the development is "to join, to fasten, to found, to establish". From here we get the word musad מוסד - "establishment, institution", well known as the Mossad - Israel's intelligence agency (institution).
Steinberg has a similar theory to Jastrow - also connecting sod, sad and yesod. However, unlike Jastrow, who writes that the common root means "to join", Steinberg feels it means "to establish on a base, to sit in a place". Therefore the original meaning of sod was "sitting together of friends". He also connects the word sadin סדין to the same root. This I have trouble with. I can put aside the fact that current research says that sadin derives from a Sumerian word, since I'm guessing Steinberg didn't have access to that information. However, while in Mishnaic (and current) Hebrew sadin means "sheet", in Biblical Hebrew sadin meant "garment". His attempts to show that based on Mishnaic use the word originally meant "sheet" (which covers the the base where one sleeps) is a bit of a stretch.
However, Steinberg does present a theory which I did find some more modern support. He writes that the root סוד - meaning "to establish, sit" has Indo-European cognates, in the Sanskrit sad, the Latin sedere, and the English "sit". He's referring to the base *sed, which is the source of many related words in English. Starostin connects the IE root here to our Semitic root here (although if I follow it correctly, only to yesod, not sod.) Of course all the normal caveats apply to the Nostratic hypothesis, but Steinberg might have been on to something.
So we can see from here that if sod originally meant "secret", then it might be related to "soda". If it originally meant "assembly", then there's a chance it's related to "sit". But there's almost no possibility it's related to both.
Another thing to rule out is my own theory from before I started researching this post. I thought perhaps the original root meant "to cover up" - which would have connected it to the Biblical word sid שיד - "whitewash". In Talmudic Hebrew the root becomes סוד, but no one makes any connection to our sod. I did even find a theory that the town Sedom (Sodom) סדום is related to שיד, as it was found in Emek HaSidim עמק השדים.
Had that theory worked out, I would have been able to connect sawad - black to sid - white, via sod. But while that connection didn't work out, if you'd really like to connect black to white - you can read this post from almost two years ago that did the same thing...