What, exactly, is meant by the threat, “I’ll slap you like a redhead stepchild”?
And what does the poor kid’s hair color have to do with it?
Red hair is certainly attractive on many adults. However, there is a certain look that some redheaded children have that makes them stand out among other people. Think bright red/orange hair with freckles and pale skin.
Combine that with long-standing stereotype of stepparents abusing their stepchildren (Cinderella) and you can see the imagery of a brutal stepparent singling out the child that looks different.
At one time and in certain quarters, red hair was considered highly undesirable and a stepchild was more of a burden than a loved relation. Think negative synergy. 
I am a redheaded stepchild, and I agree with Shagnasty and Carnac. The saying is combining two stereotypes for “outcast child.”
When I was a youngster in the '50s, the saying sometimes had an additional cruel element to it. I remember hearing “as welcome as a redheaded, crippled stepchild.” Sensitivity to the feelings of the handicapped has caused this form of the saying to become quite rare.
Thanks - that all makes sense. pinkfreud, I’ve never heard that version before.
Why and where was red hair undesirable? I looooooooove red hair! That may just be my Scottish blood combined with the fact that I lost my virginity to a redhead, though I’ve also got redheads in my family, though none of them are stepchildren. My grandmother, my aunt, my cousin - wait, my cousin was a stepchild - and my niece.
:smack: :smack: :smack: :smack:
IMO, I think it also meant that even if you *thought * the kid was yours, it’s not, because it’s got red hair and you don’t. I’d always heard it as “a red-headed child”, leaving out the stepchild part. That part was assumed.