According to Setsumon-kaiji (Shuowen Jiezi) (Explaining Simple and Analyzing Compound Characters), So is defined as "Tax levied on rice crops," which was originally Denso, and was collected nominally for ritual expenses.
説文解字には「田賦なり」とあり、元々は田からの徴税である田租（でんそ）をさし、祭祀の費用としての徴収を名目としていた。