This is from Batman 567, AKA Mark of Cain 1, AKA Cassandra’s first appearance.
Barbara gave Cass a home and a name. She also gave her a voice.
We don’t know, really, that this is the first word Barbara taught her, but it would certainly make sense if it was. Cass doesn’t need to tell most people to stop, really - she can just stop them - but that’s not the point. Barbara gave her a language other than violence.
It’s Cassandra’s choice how to use it - and she uses it to save a life.
Cain’s reaction, too, is telling. One can argue that he has no right to those tears - he’s the reason she couldn’t speak in the first place. It still humanises him a bit, though, and I think that’s important. It doesn’t lessen or excuse his abuse - it shows that parents who love their children can still do horrible things to them.
I wish we’d gotten some real interaction between Cain, the biological father who turned her into a weapon, and Barbara, the found mother who helped her learn to be a person.