The origin and definition of taboo
A taboo is something which is banned for religious or cultural reasons. The word comes from the Tongan word tabu and was introduced to English by the explorer James Cook who discovered the group of islands in the South Pacific Ocean.
No single taboo is enforced worldwide; things which are unthinkable in one society are permitted elsewhere. For example, cannibalism - the eating of human flesh - still happens in small pockets of society around the world, while it is unacceptable in most places.
In some countries, taboos are enforced by laws, such as the dress codes introduced in strict Muslim countries. Breaking these taboos can lead to severe penalties, from banishment to death. But, in many cases, they merely cause a sense of shame.
Taboos often reflect the history of a culture. For example, in some cultures, it is considered acceptable for a widow to marry her brother-in-law. This would make sense in a society where women are vulnerable, men are scarce due to wars or disease, and the population is sparse. But in modern or Western societies, it causes social discomfort and seems to reflect less consideration for women's rights.
In ancient China, women's feet were broken to make them appear smaller and more attractive. These days it would be unthinkable to consider inflicting such pain on a child. But, in those days, it was the norm, and girls who had not had their feet bound would stand less chance of marrying well.