Excerpted from the Wikipedia entry Maternal Insult.

A maternal insult (also referred to as a “yo mama” joke) is a reference to a person’s mother through the use of phrases such as “your mother” or other regional variants, frequently used to insult the target by way of their mother. Used as an insult, “your mother ...” preys on widespread sentiments of filial piety, making the insult particularly and globally offensive. “Your mother” can be combined with most types of insults, although suggestions of promiscuity are particularly common. Insults based on obesity, height, hairiness, laziness, incest, age, race, poverty, poor hygiene, unattractiveness or stupidity may also be used. Compared to other types of insults, “your mother” insults are especially likely to incite violence. Slang variants such as “yo mama,” “yo momma,” “yer ma,” “ya mum,” “your mum” or “your mom” are sometimes used, depending on the local dialect.

Although the phrase has a long history of including a description portion, such as the old “your mother wears combat boots,” the phrase “yo mama” by itself, without any qualifiers, has become commonly used as an all-purpose insult or an expression of defiance.

HISTORIC EXAMPLES

In the Bible, King Joram is greeted by the rebel Jehu with a hostile expression concerning Joram’s mother:
When Joram saw Jehu, he said, “Is it peace, Jehu?”
And he answered, “What peace, so long as the harlotries of your mother Jezebel and her
witchcrafts are so many?”
William Shakespeare used such a device in Act I Scene I of
, implying that a character’s mother is a “bitch”:
Painter: “Y’are a dog.”
Apemantus: “Thy mother’s of my generation. What’s she, if I be a dog?”
Also in Act IV, Scene II of
, Aaron taunts his lover’s sons:
Demetrius: “Villain, what hast thou done?”
Aaron: “That which thou canst not undo.”
Chiron: “Thou hast undone our mother.”
Aaron: “Villain, I have done thy mother.”