It's not an abbreviation at all. It's a term borrowed from chemistry, which might explain why my ex-chemistry-teacher dad knew immediately that it meant "opposite of trans". That's what it means in chemistry, and that's what it means when trans people use it.
So, am I cis?
To borrow from Asher, when you were born, someone weighed you, noted your Apgar score, and also had a squint at your genitals. They then wrote down a M or a F on the birth record1. If you think that the M or the F they wrote down is a pretty fair description of what you are, all things considered, then you're cis. If you think they got it wrong, you're trans.
Why do you need a word for people who aren't trans?
Because calling them all "Hey You" is rude?
In all seriousness? There are always going to be times when we have to talk about how the trans experience isn't the same as the cis experience. We need some kind of word to use for those conversations. "Not-trans" is clunky, and marks trans people out as the strange ones. "Normal" suggests we're abnormal. "Biological" and "genetic" suggest we're, I don't know, robots or something. Cis and trans make a matching pair, on an equal level with each other.
I don't like being called cis.
Well, I don't like being called freakish, deviant, deceptive, confused, "it"... Cis strikes me as incredibly mild in comparison. But if you hate it so much, try to come up with a better expression. If it sounds nicely balanced and doesn't insult trans people, it might catch on.
What is "die cis scum" all about?
As I understand it, it's about trans people getting incredibly frustrated with the idea that we have to keep patiently explaining ourselves over and over to hostile cis people in the hopes that they will eventually see the light and grant us a bit of human dignity. When you're being asked on a regular basis to "understand the point of view" of people who think you have no right to exist, the urge to blow off some steam becomes irresistible.
It seems a bit harsh to me.
I kind of understand, because if it was a group of POC saying "Die white scum" I might feel a bit upset too. Getting defensive, though, doesn't help matters. It gives the impression that we care more about white/cis people's comfort than POC/trans people's struggles, which is not exactly the best way to prove our allyship. If it makes you feel better, tell yourself the "scum" refers to the specific cis people who need to die and that you, being a good friend to your trans fellow travellers, aren't in that group2.
How is "die cis scum" any different from "die trans scum"?
Because only one of them is happening on a regular basis. I think I heard once about a trans woman who killed a cis person for being massively cissexist. And you bet she was arrested immediately and subjected to the harshest ravages of the prison system. Meanwhile, the Transgender Day of Remembrance site lists name after name of trans people who were murdered, misgendered in death, even blamed for their own murders through the disgusting "trans panic" defence.
Beyond that, cis people pass and vote for laws that make trans lives unbearable. Cis people propagate the idea that trans people are inherently criminal, deceptive, unstable, untrustworthy and harmful. Cis people box trans people into a corner so the only possible escape is to take our own lives. Cis people are already killing trans people, directly and indirectly, all the time. Trans people are just muttering "die cis scum" when the frustration gets too much. I think we're being remarkably restrained, personally.
1The situation is slightly more complicated for intersex people, but trans people have made more than enough wide-ranging statements about intersex people so I will simply note that they are outside the scope of this post.
2If you aren't a good friend to your trans fellow travellers, sort it out instead of complaining that your cissexism gets you called nasty names.