I agree that if I am confident that I know what the sentence means (as you seem to be), that should increase my confidence that I also know what "should" means in that sentence (ditto "vote" and "pocketbook").
But I'm not confident that I know what that sentence means, propositionally anyway, and your stated reasons for such confidence (that lots of people use the sentence without noticing a problem) don't seem compelling to me, because lots of people regularly utter all kinds of sentences whose propositional content is deeply unclear. And, in particular, the ambiguity surrounding that sentence has not much to do with "vote your pocketbook" (which, while a highly metaphorical phrase, I'm pretty confident I understand) and quite a lot to do with "you should X".
How many people, after all, could come up with a taboo or definition for 'should'?
Very few. Very few could even come up with an explanation of what they mean by "should" in a particular sentence (such as "you should vote your pocket book") which is a noticeably simpler task.
That isn't somehow evidence that they know what it means. Quite the contrary.
because lots of people regularly utter all kinds of sentences whose propositional content is deeply unclear.
So, suppose a pair of construction workers, Bob and Jill.
Bob: Jill, pass me that hammer. Jill: Which one? Bob: The one I want has a black handle. Jill: I see it, here you are.
Let's posit that Bob could not taboo or define 'has'. Jill could not taboo or define 'are'. I think most people couldn't, but we might disagree on that. I think they are likely to have trouble with 'that', 'one', 'want', 'see' and 'it'.
Are you saying that the propositional c...
I find Eliezer's explanation of what "should" means to be unsatisfactory, and here's an attempt to do better. Consider the following usages of the word:
All of these seem to be sensible sentences, depending on the speaker and intended audience. #1, for example, seems a reasonable translation of what a pebblesorter would say after discovering that X = Y*Z. Some might argue for "pebblesorter::should" instead of plain "should", but it's hard to deny that we need "should" in some form to fill the blank there for a translation, and I think few people besides Eliezer would object to plain "should".
Normativity, or the idea that there's something in common about how "should" and similar words are used in different contexts, is an active area in academic philosophy. I won't try to survey the current theories, but my current thinking is that "should" usually means "better according to some shared, motivating standard or procedure of evaluation", but occasionally it can also be used to instill such a standard or procedure of evaluation in someone (such as a child) who is open to being instilled by the speaker/writer.
It seems to me that different people (including different humans) can have different motivating standards and procedures of evaluation, and apparent disagreements about "should' sentences can arise from having different standards/procedures or from disagreement about whether something is better according to a shared standard/procedure. In most areas my personal procedure of evaluation is something that might be called "doing philosophy" but many people apparently do not share this. For example a religious extremist may have been taught by their parents, teachers, or peers to follow some rigid moral code given in their holy books, and not be open to any philosophical arguments that I can offer.
Of course this isn't a fully satisfactory theory of normativity since I don't know what "philosophy" really is (and I'm not even sure it really is a thing). But it does help explain how "should" in morality might relate to "should" in other areas such as decision theory, does not require assuming that all humans ultimately share the same morality, and avoids the need for linguistic contortions such as "pebblesorter::should".