Taboos are a complicated concept. In its most watered-down sense, the word can simply mean anything that is forbidden . . . but we don’t normally say, for example, that it is taboo for a customer to go into the stock room of a store or the kitchen of a restaurant. “Taboo” means more than merely “not permitted.” It has another dimension — one that goes beyond the ordinary.
That’s due to the origin of the term. Many Polynesian societies have some word along the lines of tabu/tapu/kapu, which depending on context gets translated both in the direction of “sacred and inviolable” and “unclean and cursed.” Whether its connotation is positive or negative, there’s a distinct spiritual component to the idea. And that’s because, in a sense, taboo is about metaphysical danger.
You can see this by looking at examples of taboos, both Polynesian ones and things from elsewhere in the world that seem to fit well under that header. Places, for example, can be taboo, meaning that no one — or only specific people — are permitted to go there. You see this a lot with temples and other sacred sites, where it’s sometimes handled in a tiered fashion: some areas are open to the public, others only to members of the religion, others to only clergy, etc. The Holy of Holies could only be entered by the High Priest, and only on Yom Kippur; access to the shintai or “divine body” in a Shinto shrine is similarly restricted.
The shintai example blurs over into objects being taboo. In societies where only men are hunters, it may be taboo for women to touch their bows and arrows, spears, or other hunting weapons. There are countless food-related prohibitions, whether it’s the Polynesian forbiddance of women eating food prepared by men, the Islamic forbiddance of women and men eating together, the Hindu forbiddance of eating beef, or Cú Chulainn’s personal geas against eating dog meat. His name means “the hound of Culann;” where human lineages are thought to be descended from or blessed by certain animals, killing or eating those animals is often taboo, in much the same way that cannibalism is. It can also be taboo to hunt certain animals on certain days, or in certain seasons — which may have its roots at least partially in resource management, ensuring those species don’t get overhunted and wiped out.
People can also be made taboo, in the sense of having strict prohibitions about how you can interact with them. In a number of societies, menstruating women are considered unclean and one should not come into contact with them. The same can be true of close relatives to a recently deceased person. You can potentially group the various “untouchable” castes under this header — and then contrast that with Polynesian societies, which often featured a huge array of taboos around high-ranking individuals, such that even for them to set foot on a patch of ground would cause it to become hallowed and forbidden to the common folk, and to step on the shadow of such a person was punishable by death.
So what does this have to do with metaphysical danger? Taboo — or rather, tapu and its cognates — touches on another Polynesian concept, mana. Whole books have been written unpacking both terms, but mana is more than just the “magic juice” we’ve become accustomed to in video games. It’s spiritual power in a more general sense, which can be inherited from your lineage or gained through great deeds . . . and it can also be lost.
Taboos can therefore be viewed as measures to protect mana, and/or to protect people from it. Foci of great spiritual power could be dangerous to those who are not prepared, or they could be damaged by contact with the unworthy. Removing a taboo — rendering the focus spiritually safe — generally involves either cleansing or “decommissioning” it (as there is evidence that Mayan temples were decommissioned when their cities were abandoned). You don’t want to leave that kind of energy just lying around where it could hurt someone.
And breaking a taboo? That’s perilous. You aren’t just breaking the law; you’re violating the spiritual order of the world. It invites curses, sickness, retribution from the gods. Someone who breaks the law might be hailed as a Robin Hood or other heroic figure, but someone who breaks a taboo is much more likely to be shunned by their whole community . . . at least until people see that the expected doom does not fall upon their head. Countless people through the ages have seen their holy sites defiled by invaders, and the gods have failed to smite them for it. That kind of thing can be deadly to faith.
Because science fiction novels often leave out religion and other spiritual matters, this mostly shows up only in the form of one species unwittingly or maliciously violating the social norms of another — norms which may include things that merit the term “taboo.” Any repercussions for such actions are usually political or interpersonal, not metaphysical. But in fantasy, there’s leeway for prohibitions to be anything from a foolish superstition to a very practical safety measure. Me, I tend to assume something like the second mode until told otherwise, because I like fantasy where the baseline rules of scientific western cosmology don’t necessarily apply.
Whether it’s “real” or not, though, what I always want to see from this kind of thing is the feeling that the taboo means something. Sure, sometimes it’s just a traditional practice people observe without really believing in it — but how much more force does it carry when the characters have a real, visceral fear of what will happen should the taboo be broken? The former just elicits a shrug; the latter is a powerful emotional moment. And, depending on your worldbuilding choices, a powerful spiritual one as well. Or even if the taboo doesn’t get broken, seeing the lengths the characters go to in order to preserve it can take the story in some interesting new directions.