As the article details, the invocation of maternal incest is ancient and wide-ranging.
While it's not quite a universal insult, variations on the command to commit incest with one's mother appear in every region of the globe. Anthropologists note that, across cultures, the most severe insults tend to involve a few basic themes: your opponent's family, your opponent's religion, sex, and scatology. Because motherfucker covers two of these topics-plus incest, a nearly global taboo-it's a popular choice just about everywhere.
The Explainer provides rampant usage in Mediterranean, Chinese, African and Arabic culture. In America, the usage is more recent. "The phrase was considered so vile in late 19th-century America that, in another Texas court case, it was argued that a man who had been called a "mother-fucking son-of-a-bitch" by a person he later shot "could not be found guilty of a higher offense than manslaughter," so grave was the offense." And even in these less sensitive times, where we rarely think about the words' meaning, it's an insult of high order.
The insult, as the article says, plays on some of the most fundamental taboos, while also managing to insult the target's mother's virtue, infantalize the insultee, imply only a mother would have him, and, while Slate doesn't suggest it, probably play on some deep-help Madonna perceptions as well, adding sacrilege to the mix. You'd think adding a father to the brew would render it even more potent, surely, and that's one thing I wish the article had addressed, too - why no daddy issues? And do we still hold motherhood in more reverence than we know? It might go some ways towards explaining the bile directed at Madonna and Jesus - the material ones, that is.