When you are someone’s guest, you’d better be a good one, or you could find yourself engaging in hostilities.
Four thousand years ago, the proto-Indo-European (PIE) language existed somewhere in eastern Europe, and it gradually spread and evolved to form many languages all across Europe and parts of Asia. PIE was the source of many modern English words, and when we trace them back, comparing them with related words in other languages, we can learn a lot about the nuances that grew out of those original words.
Back when PIE first existed, if you talked about a host welcoming a guest, you used the same word for both people—a word something like *ghosti-. This use of the word, for both host and guest, came down into many of the languages that later evolved from PIE.
But why would any language use the same word for both the guest and the host functions? Wouldn’t that get awfully…confusing? Not necessarily, if we consider the fact that the host-guest relationship was presumed and expected to be a reciprocal one. That is, if you were a host who welcomed a guest into your home, you had an ethical obligation to treat that person properly, because one day, you might also be a guest in someone’s home, and you would want to be treated the same way. So the reverse was also true: if you were a guest, you had better treat your host well, or there could be dire consequences.
Think back to one of the great Greek epics, Homer’s Iliad. Remember what started the Trojan War? The young man named Paris, from Troy, was a visitor—a guest—in the home of Menelaus, one of the rulers of Mycenae. But Paris ended up running off with Helen, his host’s wife, taking her back to Troy with him. This wasn’t only a matter of adultery—this was a violation of the sacred guest-host relationship. And as in many ancient cultures, this was a very big no-no!
It wasn’t a minor thing that one of the names of Zeus, the head Greek god, was Zeus Xenia (where xenia was a Greek word that was also derived from the PIE word, *ghosti-). The role of Zeus in this aspect of his godhood was to protect the sacred nature of this guest-host relationship.
Therefore, it’s no wonder that there were hostilities, when Menelaus and his brother brought armies to Troy and attacked that city, resulting in the Trojan War—involving several army hosts—which would have landed a great many people in hospital (if such places had existed then).
Yes, those three words—hostilities, hospital, and even host, this time meaning a large military group—also come from exactly the same proto-Indo-European root as host, guest, and xenia. These three new words, though, came into English through Latin, another language that had descended from the original PIE language. In Latin, the word hostis (also descended from the PIE word *ghosti-), meant “stranger” in its earliest usage. And a stranger was someone who could very well need your hospitality (yep, this word comes from the same Latin root, which also came from that original PIE word). But this person, this stranger, could just as likely be an enemy. And in the end, that’s the meaning that the Latin word hostis finally settled on.
Meanwhile, if you were automatically hostile towards someone for no other reason than that he or she was a stranger—then you were xenophobic (and there it is again, xeno, another word derived from the PIE word *ghosti-).
So what does all this amount to, with all these words that have descended from the original PIE word,*ghosti-? Well, it all seems pretty fraught and complicated, but it’s actually quite clear. Whether you’re a host or a guest, just be hospitable, and nobody will get hostile.
(Many thanks to the great research efforts of Kevin Stroud, who I benefited from, in part, in writing this post. If you want to learn more about the history of English in general, and some of these very ancient linguistic connections in particular, I highly recommend Kevin’s superb History of English Podcast.)