Rebu means taboo, forbidden, not allowed, not allowed to do something according to Karo customs. Anyone who violates, is considered not to know the custom, and was scorned by the community. Rebu in the Karo community is divided into three parties:
- Between mami (woman-in-law) and kela (son-in-law). In a narrow sense, mami is the mother of the ego's wife, in a broad sense, is the wife of the mother's brother or the ego's mother of the ego's wife). While in a narrow sense kel is the husband of the ego's daughter, in a broad sense is the son of the ego's father's sister. Before the wedding, this class is called bere-bere or where.
- Between bengkila (male-in-law) and permain (daughter-in-law). Bengkila in a narrow sense is the father of a woman's husband, in a broad sense the husband of a woman's father's sister. While the game in a narrow sense is the wife of the son of an ego person. In a broad sense is the daughter (including the son) of the brother's wife ego.
- Between turangku and turangku. Turangku has two meanings, first, if the ego is a man, then turangku is and means the wife of his wife's brother (in-law), second if the ego is a woman, turangku means the husband of her husband's sister (in-law). What is contested, forbidden, prohibited, not allowed, not allowed to do something according to Karo customs is:
(1) direct speaking,
(2) touching body parts,
(3) sitting face to face,
(4) sitting on a mat/chair.
The manifestation of this rebu (forbidden) in Karo customs, is that it is forbidden to talk, it is forbidden to sit in a chair, for example with in-laws who are of the opposite sex with ego, forbidden to talk to husband-in-law or wife of a different sex with ego. Rebu is a sign of the limits of self-independence, a sense of self-freedom, through this kind of behavior people remind and are aware of social principles in the way of living with kinship, then through rebu, people will be able to control their own actions. Rebu gave birth to mehangke (reluctance), from reluctance breeds respect. Respect breeds manners. This is an educational element of the Karo custom which has the nuances of preventive social control. However, in current developments, the rebu tradition tends to be ignored. It is common to see a man talking directly to his father-in-law.