I have tried to find the origin of that phrase (this may come as a shock...but I love etymology) on information sites and discussion groups for idioms, and slang expressions, and the only consensus seems to be that no one really knows exactly where it came from
. Here is a commentary by one person that seems to be as good as any:
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from
http://www.word-detective.com/042805.html
While we're on the subject, a phrase I've been also been asked about is "beat [someone] like a red-headed stepchild," meaning "to beat (either literally or metaphorically, as in a sporting contest) severely and thoroughly." This phrase, perhaps most often heard in the southern US, crops up fairly frequently in various contexts, as does "red-headed stepchild" alone in the sense of "object of neglect or discrimination" (as in "Environmental protection is the red-headed stepchild of the Mayor's administration").
Unfortunately, I've yet to find the origin of the phrase. The best guess I've seen is that while any child may face abuse or neglect from a step-parent, one with a notable feature (such as red hair) reminiscent of the departed former spouse may be a particular target in such a situation. It is also true that red-haired children are often the butt of jokes by their peers, especially in Britain (where they are called "gingers"), and in several European cultures red hair has historically been considered an unfavorable characteristic.
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