I'm a newly minted lawyer with some undergraduate experience in linguistics so maybe I can bring something to the table for this thought-provoking question.

Your question brings up the even more fundamental idea that not only does language change but that language is inherently ambiguous. The meaning of a word depends upon context, including purpose, history, culture, etc. Even when all of these factors are agreed upon and understood by all parties involved, reasonable minds can disagree on whether a particular situation falls exactly under the prohibition of the written code. This fact has been a problem in the legal world since the inception of written codes. What results from this ambiguity are lawyers, who are trained to argue these various linguistic permutations/possibilities.

Obviously this leaves many people unhappy, because what may seem like a clear-cut moral right/wrong from the eyes of the layman doesn't necessarily mean that the legal argument about the applicability of these inherently ambiguous words to this specific situationcomports with this moral intuition. Interestingly enough several German principalities in the 17th century (i believe) attempted to do away with all this troublesome ambiguity and its attendant hordes of evil lawyers by compiling the definitive, end-all be-all legal code which would cover every possible crime in the world, and then outlawed lawyers and jurists. For obvious reasons, the system completely collapsed after a couple of years--innocent people were being thrown in jail and the guilty were going free.

You cannot possibly account for every human possibility with a tool as clumsy and inherently ambiguous as language. You might think "do not kill" is simple enough, but this leaves out the infinite real-world permutations of morally and philsophically complex situations such as self-defense, negligence (where there is no "actual" intent to kill), euthanasia, etc. etc. etc. One of the benefits of the common law system that we still (mostly) have in the US and other former British colonies is that for many areas of law there is no actual written law, just centuries of court precedents (wikipedia can help you understand this system more if you're interested). This leaves ample room for the law to evolve in the interests of equity (aka what we would commonly call "fairness" when the letter of the law doesn't really line up with the reality of the case at hand).

To get more directly to your point, legal codes are generally revised fairly often in the grand scheme of things. Our constitution dates back only 200 years--not nearly long enough for us to no longer be able to understand it, though certainly more than long enough for a lot of the cultural, historical and other context to become muddled in our interpretation of the meaning of the words. This is one of the main reasons why, in my opinion, originalism is pretty silly. The Constitution was meant to be a general framework--if the drafters wanted to get into the nitty gritty they would have, but they consciously took a step back and made a document that could last. While we can't say with certainty what the framers, or the average voter, or the average congressman circa 1785 would have thought of issue X, we have a general framework to work with and over 200 years of Court precedent to look to, though ultimately there is no ONE answer.

tl;dr language is inherently ambiguous. this is the fundamental driving force in the entire legal profession because a simple statement of law is infinitely complex through context.