There has already been some discussion about how to tag questions that are (explicitly) not specific to some discipline (e.g. this one or this one). The author of the first example question I just linked you to put forward the tag 'discipline-agnostic' to take care of this category of questions. One argument that has been brought up is that this is the way analogous types of questions are dealt with on some other Stack Exchange sites.
Personally, I don't think it's a good tag because it is hard to understand what it means; at least at first sight. I was initially under the impression that the tag implies an actual connection to agnosticism (i.e. the question was somehow related to religion), and only realized after pausing to think for a while what it really means. This is a very serious issue for a tag: The main purpose of tags is to clarify and simplify, not to confuse. I, instead, propose using an alternative. Possibilities include 'no-specific-discipline' or 'general-science'. Here, I would like to see a more in-depth discussion of the pro's and con's of each idea, as well as some fresh, new ideas if you have any :-)