to be treated like a redheaded stepchild means to be treated unfairly to be beat like a redheaded stepchild thus means to be beaten very harshly It originates from Irish immigration in the 1830-1840s. A time when red hair usually indicated irish decent. Irish people were viewed very poorly at the time and when a woman had a redheaded child from either premarital s*x or a previous marriage the child would be treated poorly by a new husband. obviously this was not in all cases but it was enough to warrant a saying.