1888, South African, "party of trekkers" (see trek). Meaning "fan of the television program 'Star Trek' " attested by 1976.
Examples from the Web for trekkie
A trekkie will, however, drop everything to discuss the merits of Klingon spirituality.‘Star Trek’ for Dummies: Get Ready for ‘Into Darkness’ With Our Primer|Sujay Kumar|May 14, 2013|DAILY BEAST
Zwillich knew the image would lay bare his Trekkie obsession to friends and followers.