Soon after South Park first aired, I happened to be hanging around a student lounge with my then-girlfriend Amy. This guy Shane was there, and in the course of various good-natured insults being thrown around, I happened to state (with an appropriate South Park accent):
your mom’s a crack whore!
He froze. His face blanched, and he ran from the room, much to the confusion of all present.
The reason for his behavior became apparent some time later, when various people went to see what was wrong. Shane’s mom really was a crack whore. Shane had been raised by his aunt and uncle; his mother was, throughout his childhood, constantly on the street, in and out of prison, psychiatric hospitals, and, from time to time, supporting her drug habit through prostitution.
Clearly, then, my insult was in grave error. According to my linguistics text (Language Development, Erika Hoff-Ginsberg, Brooks/Cole, 1997):
There are rules governing the content of these insults and their use in discourse. In terms of content, the insults must be about someone else (typically the mother), not the person the insult is addressed to. The insult typically refers to appearance, age, poverty, or sexual promiscuity, and, most important, the insult must not literally be true. Guaranteeing the falsity of insults depends in part on shared knowledge. For example, the insult that your mother is a prostitute would not be addressed to a boy whose mother really was a prostitute. Falsity is also ensured by making the insults so extreme as to be literally impossible.