The Origin of the Word "Relationship"

The word "intercourse" had that meaning (not just in the sexual sense) back then.

My guess is that relationships existed for many centuries before the word was first recorded....just a hunch

Going to the Germanic side of English, you have the word "bond". The Online Etymological Dictionary says that's connected to the word "band", which can be traced all the way to the Gothic (the oldest written Germanic language, 4th Century AD)"bandwa": "a sign". Apparently a band of cloth worn by soldiers for identification. I think "bond" is the old-fashioned synonym you couldn't find.

Synonyms from bygone times: liaison, affair, and, long ago indeed, romance.

Bryan is partly right, "relationships" were often denoted by more specific words.

I do think that "friend" is the closest synonynm from the 1600s and 1700s. Numerous quotes from Shakespeare and the Bible show a wide variety of meaning, from "patron" to "acquaintence" to "disciple" to "bosom buddy".

By considering the word "intercourse" in its older usage we see that the need for the word was limited to very abstract, highly distanced discussions. The relationships that "mattered" were more circumscribed and precise. There was no need to have a word that would encompass all of friendship, marriage, acquaintanceship, business familiarity, hierarchy, kinship and other familial interactions in the same term.

I also suspect the popularity of "relationship" was due to the pop-psychology scientistism that sought to divorce love, marriage, sex, affairs, and other romantic entanglements from their traditional connotations and make them an object of "neutral" discourse. The other terms were loaded with the baggage of traditional judgment and creating a new term became a SWPL way of signalling your modernity and neutrality. In some cases it became easier to create a moral equivalence between (for example) marriage and homosexual coupling or between platonic friendships and affairs.

It's interesting how euphemisms decay over time. "Intercourse" is used, with a straight face, in the scientific literature.

I still snicker on the inside when people say "boyfriend" or "girlfriend" because they're too squeamish to say "lover."

It seems like possibly there's a trend. I note a lot of people nowadays don't like to put labels on their relationships--are they a couple, et c. I wonder if people have a greater need to keep their options open. Getting tied down nowadays seems more costly than it did int he past, since opportunity costs are so great now compared to then!

O'Brian's Master And Commander (set around 1800) uses "particular friend".