Red headed stepchild is a popular slang colloquialism, used to refer to a mistreated person who is differentiated because of their physical appearance.
Red hair was closely associated with Irish people in the United States during the 19th and 20th centuries, when the Hibernians were receiving similar discriminatory treatment from Americans as Black people.
It is due to this fact, that a “Red headed stepchild” implied a big shame associated with the family, because one of the members had bore an “unclean” Irish stepchild, who in turn would get mistreated both by the family and everyone else, while growing up.
Although the exact origin of the phrase “Red headed stepchild” is not known, it first started appearing in its colloquial sense in newspaper articles and magazines in the 1910’s, meaning it had an already established presence in spoken contexts.
It may be traced back to the middle of the 19th century, when the mass immigration of Irishmen into the United States began, although there are no records from this era.
The most plausible explanation for the phrase is that it was conceived sometime in the second half of the 1800’s, with the spread of Irish people in the US.
“Red headed stepchild” remained a popular expression well into the 20th century, with growing Irish acceptance slowly eroding it, however not being enough for the idiom to be completely forgotten.
It is still widely recognized and used in contemporary contexts, both in media, online or real life.
“Red headed stepchild” was first defined on Urban Dictionary on December 8th, 2002, with several other entries following.