I am a big fan of demystifying churchey words, not because I am irreverent, but because I don’t want their effect to be lost. So, “holy”. I would describe “holy” as ‘designated for a particular purpose’. People have holidays (holy-days) designated for remembrance of certain religiously significant events. A holy day, or holy place, or a holy object, is simple one designated for a specific purpose. You could say that a holy people is simply a group given a particular purpose. Holiness is about purpose and focus.
So, my toothbrush is holy unto my mouth. It does not get used by anyone else, or used for cleaning the toilet or the sink; it has a particular purpose, and that purpose serves as the framework for all the decisions made about it’s use. That is holiness: a framework of purpose and focus through which all decisions are vetted and filtered.