Your mother

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, 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. Insults involving "your mother" are commonly used when playing the dozens.

Although the phrase has a long history of including a description portion (such as the old "your mother wears combat boots", which implied that one's mother worked as a prostitute in the military), the phrase "yo mama" by itself, without any qualifiers, has become commonly used as an all-purpose insult or an expression of defiance.

"Your mom", sometimes also "yo momma", generally depicts an obese, old, short, hairy, permissive, poor, unintelligent and ugly person. The phrase is usually not literally meant as a direct insult to a person's mother, but is supposed to describe a type of imaginative fantasy person with the said characteristics. When visually depicted, "your mom" is usually ugly, neglected or absurdly obese. These attributes are also seen in the intro of a maternal insult, e.g. "Your mom is so fat,...", but sometimes occur later within the joke or even deliver the punchline itself.

Sometimes, even the gender of the "mom" is doubted, which is reflected in a lacking femininity or excessive masculinity.

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Wikipedia