In this saying, it means that the person you're talking to hasn't shown their worst side yet. It's like they're saying, "You think I'm tough now? Just wait until you see me at my absolute worst." It's a way of warning someone that they haven't experienced the full extent of the person's anger or frustration.
It's like when you only see the tip of an iceberg, but there's a whole lot more hidden beneath the surface. So, when someone says this, they're telling you that you've only seen a small part of who they are, and there's a darker side that you haven't witnessed yet.
It's a kind of challenge too, like they're daring you to push them further and see what happens. So, when you hear someone say this, it's a good idea to be careful and not push their buttons too much, because you never know what might happen when you finally do see their bad side.