From Middle English lawe, lawe, from Old English lagu (“law”), taken from Old Norse LG (“law, literally “things established or firmly established), originally the plural of lag (“layer, stratum, placement in order, measure, trace), from the protoGermanic *lagą (“what is established”), from the Proto-Indo-European*. Substantive due process should be understood as a relatively recent historical development, although by no means a bad one. Sandefur's intriguing final question, I don't think it's particularly controversial that authors of legal texts sometimes make mistakes in terms of meaning. Sandefur's example of a law that taxes the importation of “fish”, as applied to dolphins, is an excellent example of the importance of context and history in determining the real original linguistic meaning (as opposed to the expected application) of legal language.
To the extent that this statement is simply intended to convey that meaning cannot necessarily be reduced to the subjective intentions or understandings of a particular, identifiable speaker or group of speakers, I do not necessarily disagree. To take an example suggested by Professor Lawrence Solum, the word “deer” in Middle English was used to refer generically to all animals of any type, rather than as a reference to the particular animal that contemporary English speakers use the word “deer” to identify. Sandefur states that in 1791 there was a widely shared public understanding among Americans that understood that “due process of law” or the “law of the country” encompassed abstract protection against all forms of arbitrary government decision-making. I think there is a lack of evidence to support such a proposition for the reasons explained in my original answer.
This is because the term “contagious” had an objective meaning that was independent of the erroneous expectation of how the law would apply to a particular disease. Sandefur wants to suggest that there is an objective “meaning” of constitutional language that can be totally divorced from the subjective understanding of some group of speakers and listeners, so I think that the point of view is fundamentally wrong. More generally, I think that most modern originalists would agree that the expectations of the enactors' about how a particular text will be applied in a given factual context are not necessarily determinant of the meaning of that text. It is more common for Indo-European languages to use different words for a specific law and for law in the general sense of institution or body of laws, for example, in Latin lex a ley, ius a Derecho, especially legal right, law.
The words for law in the general sense mainly etymologically mean what is right and are often related to adjectives that mean right (in themselves, often figurative uses of words for straight, straight, true, adequate, or use, custom). The Indo-European words for a law most commonly come from verbs that mean to put, place, establish, place, place, place, place, such as the Greek thesmos (from tithemi to put, place, place), in old English dom (from STANDING *dhe- to put, place, establish), Lithuanian statins (from statyti cause to stand, set) and Polish ustawa (from stać stand).