A
colloquialism meaning "a really
severe beat-down." Apparently it comes from some people's
belief that
redheads and
stepchildren ain't worth much, and if you combine the two, you're getting something that you can safely
wallop with
maximum force and enthusiasm.
Addendum: smartalix says: "I believe the term indicates more that the child is more visible as 'not mine' due to the
red hair, making it a target as 'not really' part of the family."