Red-headedness is a rare characteristic, and can only be present if one parent also has this passive gene. A "red headed stepchild" would be a constant reminder that they were not your natural offspring, drawing attention to themselves and distinguishing themselves from the other children. Perhaps the stereotype of redheads being fiery and independent played into this as well - the one redhead would be seen as the troublemaker of the bunch.

It means that because the child has red hair, he has no soul. The thing is that's only part of it, because a step child is someone that you don't have to love, but may one day. If they have a soul. The fact that it is a child that has no soul who you have no real attachment to means that unscrupulous things can be done to the boy without any need to feel guilty.

"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 whallop 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."" Source and further information: http://everything2.com/title/beat+like+a+redheaded+stepchild "Wondering whether this could be a reference to the idea that such a kid may not be a natural son, but the child of an incubus or a changeling. A red-haired child born to parents both not having red hair, in a population were such occurrences would have been rare as the necessary genes were, well before the time of Mendelian genetics - how would that not have been considered as remarkable at least? Beating the hell out of a changeling was said to sometimes make the real kid take its place again, I think." Source and further information: http://forum.wordreference.com/showthread.php?t=61743&page=3 Further information: http://books.google.com/books?id=i33BWgxbvXgC&pg=PA522&lpg=PA522&dq=beat+like+a+red-headed+stepchild&source=bl&ots=QvsS2ULrK-&sig=rgZ8T1TrlJRoB9WUynXynRbMXLk&hl=en&ei=8bbhSru7AYee_gatpaz-AQ&sa=X&oi=book_result&ct=result&resnum=5&ved=0CBsQ6AEwBDgK#v=onepage&q=beat%20like%20a%20red-headed%20stepchild&f=false http://www.urbandictionary.com/define.php?term=red+headed+stepchild http://www.urbandictionary.com/define.php?term=beat%20like%20a%20red%20headed%20step%20child