'cis women' is not a personal attack. Here, 'the truth' matters more than respecting others' feelings: I would rather use standard terminology that only means and connotes 'non-trans' for the majority of people (and is what I intend it to) rather than use politically sided language.
If you hadn't told us that you're a teenager, I would have guessed you were very young from this statement.
No, 'cis' means absolutely nothing to the majority of the population. I only came across it myself in this particular context about 10 years ago. Most people who have heard of it would only know it from its scientific context or because they've studied Latin. It is about as far removed from standard terminology as it could get.
A large proportion of people think that a 'trans woman' is a woman who wants to be a man. Very young people like you have a very skewed idea of most people's knowledge of these issues. We're a bit of an exception on here because many of us have been discussing this for years.
You might think that 'cis' is standard terminology because you're not old enough to remember a time (only about 10 years ago) when it was only used in chemistry or by Latin scholars.
Earlier in the thread I posted an archive link to the talk guidelines for this part of mumsnet which were brought in 10 years ago. This is what they said concerning the use of 'cis':
https://archive.ph/XbZ8U
That said, it’s clear that most trans people find the use of pronouns or names that they or others have consciously rejected, to be hurtful and would therefore struggle to engage in a discussion with those who insist on using them. The same is true of the expression ‘Trans-Identified Male’ or ‘TIM’. Likewise, many feminists are affronted by the term ‘cis’ and ‘terf’, so using these terms will make civil debate less likely. As we’ve said, context is everything – but our moderation team will most likely delete these expressions if we feel they're being used in a deliberately inflammatory way.
Even back then it was recognised as an offensive term, and it's no different today.