In many Australian Aboriginal cultures, as well as among some North American Indian and African Bantu peoples, human interaction and speech between certain relatives are taboo.

This usually means total avoidance of each other, but often also involves a special parallel language that is used if necessary, for instance if one member of the tribe is speaking and is aware that a ‘taboo’ person can hear what is being said.

These special languages are often known as ‘avoidance speech’ or ‘avoidance language’.

Because a very common taboo is the interaction between in-laws of opposite sex, the phenomenon is often called ‘mother-in-law speech’.

However, such taboos are not restricted to in-laws. For instance, a widespread no-no amongst Australian Aborigines is any kind of contact or interaction between brothers and sisters once they become adults.

The nature of the taboo varies, but often involves complete avoidance of the other person.