It’s been used for centuries, just not necessarily for orientation. It comes from Latin, just like “trans” does.

https://www.mcgill.ca/oss/article/history-general-science/word-cisgender-has-scientific-roots

Cis is a Latin prefix that is the opposite of the Latin prefix trans, so it makes sense to use it for people who identify as their gender assigned at birth.

Somehow I don’t think the shadow council of women is entirely responsible for this term.

Why just limit your question to cis woman when there are also cis man?

The prefixes “cis” and “trans” are from Latin: “this side of” and “the other side of”, they are opposites and have been used in chemistry for decades.

Because they are word roots. “trans-” means on the other side of something like “transatlantic” while “cis-” means on the same side like cisatlantic.

Probably somewhere around the time they realized referring to themselves as “actual women” was not a particularly inclusive, or kind, look.