Origins of "cis" and "cishet"?

What does the word even mean? What is it short for? Where did people come up with the word?

I know the definition (identify as the sex you were assigned), but what does "cis" mean? I never liked that word because it sounds dirty and like a slur to me but that might be just me. I dislike people calling me cis ☹️ 😕but maybe if I knew the meaning I wouldn't care as much..so I'm asking the experts..thanks!