From cis(gender) + -y, on the analogy of tranny and punning on sissy.
Meanings
(derogatory, humorous, often, rare, slang, sometimes) A cisgender or cissexual person.
"She could be a straight cissy who just had a bunch of plastic surgery done," Tabitha said. A cissy was a 'cisgendered' person, someone who wasn't transsexual, someone happy with being and dressing like whatever gender they were born as. 2003, Brian St.Claire-King, Fates Worse Than Death: Sunrise Hotel, →ISBN, page 104:
> Cis- is a Latin prefix meaning "on this side" or, less correctly, "near", as opposed to trans-, Latin for "across" or "through".
Gee, I guess that makes most folks cissies! :-) 1998 June 10, Karen Ross, “Re: what does cisgendered stand for anyway ...”, in alt.support.srs (Usenet):
If a coalition of queer and trans-rights groups came together and called for a big demonstration outside the Fox News studeios in midtown Manhattan, I would be there along with tons of other gay, lesbian, straight and bi cissies. How about it? 2014 January 8-15, Dan Savage, in Philadelphia Weekly