Seeing some tumblr posts between Facebook and reddit....I still have no idea what it is.

FROM THE URBAN DICTIONARY: Cis

Short for cis-gendered, meaning someone who identifies with the sex they were born as.

Typically used by whiny tumblr users who complain about not being accepted for who they are and yet bash these "cis" people for being born and being okay with the sex they were born with.

Example: I am a female to male transgender and I hate white straight "CIS" men.

It's not rocket science: it's the opposite of transgender. The only reason the word exists is because it's sometimes offensive to call something "normal" - you wouldn't like to be told that everyone else is normal and you're not - but literally all it means is someone who isn't transgender. From the Latin "cis" and "trans" meaning "on this side" and "across the other side" respectively.