I don’t get why cis people are so amused with the term cis. They sometimes say it in a very mocking way. It’s the proper Latin opposite of trans. Cis means “on the same side” and trans means “on the opposite side.” Trans people didn’t make this up, it appears in other places. You see it in organic chemistry, there are cis isomers with molecules on the same side of the carbon chain and trans isomers with molecules on opposite sides. The West Bank used to be called the Cisjordan, because it was on the same side of the Jordan river as Jerusalem. And conversely, what is now the nation of Jordan was called Transjordan or Transjordanian Palestine. It just goes to show that the people saying things like “SJWs use made up words” are never going to be satisfied, because it doesn’t get any more official than Latin.
To the anon asking why cisgender is a term: 'cis' is just the opposite of 'trans' no matter if its related to gender or not! Cis is just latin for "on this side of" and trans is latin for "on the other side of." A good example of this is the words 'cismontane' (meaning on this side of the mountain) and 'transmontane' (meaning on the other side of the mountain. Same with cisatlantic and transatlantic. So any word with a prefix of 'trans' has a theoretical opposite starting with 'cis'!