What is the difference between a Trekker and a Trekkie? extolls the difference, but I am wanting to know how the term was first used, or coined? It seems to go a long way back, further than I had assumed.
Try as I might, it was very difficult to find a definitive source that absolutely confirms where the term Trekkie originated. According to Grinnell College, in an article detailing the history of Trekkies,
Trekkie is a term that originated around 1970 as a way to describe fans of the TV show Star Trek who go above and beyond in their expression of their admiration for the show.
Wikitionary also states that the word originated in 1970 as a combination of the words trek and groupie. However, the website Fanlore states that the earliest usage of the term came in the 1968 essay A Mid-Spring's Night's Dream, or, Journey to Backstage by Ruth Berman and Dorothy Jones, which described a group of fans meeting Mark Lenard, who played Sarek in The Original Series. Both women use the term once (emphasis mine):
[Ruth] I don't know about other people, but I'm afraid I was acting out of snobbery in cataloguing the roles I'd soon Mr. Lenard play when I sent him a fan letter—I'd wanted to make it clear I wasn't just a trekkie in love with Spock and therefore with all things Vulcan, especially Spock's father, but rather a sophisticated, mature admirer of good acting wherever it appears. Which I hope is true — but I'm in love with Sarek anyway.
[Dorothy] His great, warm brown eyes are guaranteed to melt any trekkie into a helpless pool of protoplasm.
Finally, Wikipedia and a few other sites say that sci-fi editor Arthur Saha came up with the term after seeing a few Star Trek fans at the 25th World Science Fiction Convention wearing pointy ears. He apparently used this in a TV Guide interview with Pete Hamill, but I could not for the life of me find that interview. Therefore, the simple answer is we don't know but if anyone could track down that TV Guide, perhaps there would be a definitive answer.