Aspersions upon a person's mother

"Displays containing some words--odious racial epithets, for example--would be prohibited to proponents of all views. But "fighting wor ds" that do not themselves invoke race, color, cr eed, religion, or gender--aspersions upon a person's mother, for example--would seemingly be usable ad libitum in the placards of those arguing in favor of racial, color, etc. tolerance and equality, but could not be used by that speaker's opponents." - R.A.V. v. City of St. Paul, 505 U.S. 377 (1992)

I don't understand the saliency of aspersions upon a person's mother. Who invented mom jokes, anyhow? And why?