First appearance of the word "polyamorist": 1953!
Word person that I am, I've had this thing for tracking it down. Especially because the multi-love movement was hobbled for three decades by lack of a clear name for itself before settling on "polyamory" in the early 1990s1.
The usual story is that the word was invented by Morning Glory Zell-Ravenheart in her article "A Bouquet of Lovers" in the Beltane (Spring) 1990 issue of the Neo-Pagan magazine Green Egg. She only introduced the form "polyamorous" there, but her partner Oberon Zell-Ravenheart recently wrote me to say that the two of them did use "polyamory" in a glossary that they handed out at a "Polycon" convention at UC/Berkeley in August 1990 (elsewhere he says 1991), where they were featured presenters. However, in 2006 the editors of the Oxford English Dictionary (despite knowing about Morning Glory and corresponding with her) assigned priority for the word to Jennifer L. Wesp, for when she created the Usenet newsgroup alt.polyamory in May 1992. Read more here: "Polyamory" enters the Oxford English Dictionary.
Another piece of conventional wisdom is that the word was used occasionally since the 1960s. But I never found a single example and assumed that people were just misremembering "polyfidelity" (coined by "Even Eve" Furchgott in the Kerista community in the 1970s).
Well, Google Books is an amazing thing. Now you can search for words in some 15 million scanned books (out of the estimated 130 million unique books in the world) plus lots of serials, published since ever.
So I searched from Jan. 1, 1400, to Dec. 31, 1991, and found:
● polyamory, polyamoury, poly amory, or poly amoury: Zero occurrences (not counting one later book that has a typo in its publication year).
● polyamorist, polyamourist, poly amorist, or poly amourist: One, in editions of a book starting in 1953! Here it is — in the Illustrated History of English Literature, Volume 1 by Alfred Charles Ward (aka A. C. Ward). Excerpt:
If Henry VIII had not been a determined polyamorist to whom divorce or some more drastic means to annulment of marriage was a recurrent necessity, the break with Rome would probably not have come in his reign, [Thomas] More and others would have died naturally, and the whole of subsequent English history might well have been different.
(Ward published similar histories of English literature before and after 1953, and this passage was reprinted in one from 1958. The 1953 book may be a republication of earlier material for all I know, but my Google Books search did not hit on these. Note that Google Books results are imperfect and often change.)
● polyamorous, polyamourous, poly amorous, or poly amourous: Seven hits, from 1969 to 1989. Here they are. The first of these is in the 1969 novel Hind's Kidnap: a pastoral on familiar airs by Joseph McElroy (who's still writing as of 2010). Excerpt:
...Maddy disqualifying John Plante, "You have to conclude the Family quote unquote is finished as a viable socio-entity because you're committed to your polyamorous roller tribe, so you can't even so to speak let me into court." Occupying, taking over, stealing me and my flat while I shook too much chervil into the eggs, pretty too....
That's all of it that's online, per Google's copyright arrangements.
Most of these just seem to be cute, one-off wordplays, not references to any kind of movement or philosophy as we use the word today.
Update May 2011: A use in 1921 in Italian has been found; see comment #15 below, by Julio.
1 Before the early 1990s people, including me, floundered with such awkward mouthfuls as "utopian swinging" (now there's a contradiction in terms), "modern polygamy", "waterbrotherhood" (per Stranger in a Strange Land), "polymorphous perversity" (per Sigmund Freud), "synergamy" (per Robert Rimmer), and "the Harrad Experiment lifestyle."