The term originally characterized farmers that had a red neck, caused by sunburn from long hours working in the fields. A citation from 1893 provides a definition as “poorer inhabitants of the rural districts … men who work in the field, as a matter of course, generally have their skin stained red and burnt by the sun, and especially is this true of the back of their necks”.[12] Hats were usually worn and they protected that wearer’s head from the sun, but also provided psychological protection by shading the face from close scrutiny.[13] The back of the neck however was more exposed to the sun and allowed closer scrutiny about the person’s background in the same way callused working hands could not be easily covered.
By 1900, “rednecks” was in common use to designate the political factions inside the Democratic Party comprising poor white farmers in the South.[14] The same group was also often called the “wool hat boys” (for they opposed the rich men, who wore expensive silk hats). A newspaper notice in Mississippi in August 1891 called on rednecks to rally at the polls at the upcoming primary election:[15]
I live in a part of America where it’s repeated constantly to school kids. Also, that’s not actually the origin; the red necks were militant unionists who fought for collective bargaining rights in the Coal Wars, the part of American history that never gets taught.
Other sources were already posted in the comments showing that “redneck” referring to farmers predates its usage to refer to coal miners/communists. Definitely agree there’s a lot of stuff conveniently left out of our history books, though.