Wow, she's got some nerve, doesn't she?

I don't know if this is the right place, but I'm not sure where else I could ask this question. Does anyone know the origin of "cis"? I agree there needs to be a word for people who aren't trans ('cause that gets really annoying to type repeatedly :) ) but why "cis"? It doesn't even sound like a real word. Is it an acronym or did someone just invent it & it caught on?