I was first exposed to the term back in the late '90s when my girlfriend at the time was getting her Masters in social work. It was the term used within the trans community used to describe individuals whose gender identity and sexual preferences were normative - in discussions where minority gender and sexual preferences were the subject. Gender studies ain't my expertise, but back in those halcyon days the intention was that "straight" was everybody that wasn't "queer" or "LGBT" and "cisgendered" were those people in your transgender discussion that didn't need their gender explained. I can't say for sure, but I suspect that had you asked back then, nobody would expect the average person to wrap their heads around being "cisgendered" unless they were comparing themselves to people who weren't.