Red-headed stepchild.

What does this mean?

A stepchild might be singled out for abuse. But a red-headed stepchild (who presumably looks like his or her absent birth-parent) might be abused even more because he or she is so obviously different from the other children.

The expression I have heard is, "I'm going to beat you like a red-headed stepchild."

Sad, isn't it?

Only slightly more humane is: "beat you like a rented mule."