A taboo is a vehement prohibition of an action based on the belief that such behaviour is either too sacred or too accursed for ordinary individuals to undertake. Such prohibitions are present in virtually all societies. The word has been somewhat expanded in the social sciences to strong prohibitions relating to any area of human activity or custom that's sacred or forbidden based on moral judgement and religious beliefs. "Breaking a taboo" is usually considered objectionable by society in general, not merely a subset of a culture.
The term "taboo" comes from the Tongan tapu or Fijian tabu ("prohibited", "disallowed", "forbidden"), related among others to the Maori tapu, Hawaiian kapu, Malagasy fady. Its English use dates to 1777 when the British explorer James Cook visited Tonga, and referred to the Tongans' use of the term "taboo" for "any thing is forbidden to be eaten, or made use of". He wrote:
Not one of them would sit down, or eat a bit of any thing.... On expressing my surprise at this, they were all taboo, as they said; which word has a quite comprehensive meaning; but, in general, signifies that a thing is forbidden.
The term was translated to him as "consecrated, inviolable, forbidden, unclean or cursed". Tabu itself has been derived from alleged Tongan morphemes ta ("mark") and bu ("especially"), but this might be a folk etymology (note that Tongan doesn't actually have a phoneme /b/), and tapu is usually treated as a unitary, non-compound word inherited from Proto-Polynesian *tapu, in turn inherited from Proto-Oceanic *tabu, with the reconstructed meaning "sacred, forbidden". In its current use on Tonga, the word tapu means "sacred" or "holy", often in the sense of being restricted or protected by custom or law. On the main island, the word is often appended to the end of "Tonga" as Tongatapu, here meaning "Sacred South" rather than "Forbidden South".
Sigmund Freud posited that incest and patricide were the only two universal taboos and formed the basis of civilization. Notwithstanding although cannibalism, in-group murder, and incest are taboo in the majority of societies, marriages between brothers and sisters occurred in Roman Egypt.
Common taboos involve restrictions or ritual regulation of killing and hunting; sex and sexual relationships (primarily incest, necrophilia, miscegenation, adultery, fornication, pedophilia, homosexuality, intermarriage, bestiality, and masturbation); reproduction (abortion, infanticide); the dead and their graves; as well as food and dining (primarily cannibalism and dietary laws such as vegetarianism, kashrut, and halal) or religious (haram). In Madagascar, a strong code of taboos, known as fady, constantly change and are formed from new experiences. Each region, village or tribe might have its own fady.
The word "taboo" gained popularity at times, with a few scholars looking for ways to apply it where additional English words had previously been applied. For example, J. M. Powis Smith, in his "The American Bible" (editor's preface 1927), used "taboo" occasionally in relation to Israel's Tabernacle and ceremonial laws, including Exodus 30:36, 29:37; Numbers 16:37,38; Deuteronomy 22:9, Isaiah 65:5, Ezekiel 44:19 and 46:20.
Communist and materialist theorists have argued that taboos can be used to reveal the histories of societies when additional records are lacking. Marvin Harris particularly endeavoured to explain taboos as a consequence of ecologic and economic conditions.
Changing social customs and standards additionally create new taboos, such as bans on slavery; extension of the pedophilia taboo to ephebophilia; prohibitions on alcohol, tobacco, or psychopharmaceutical consumption (particularly among pregnant women); and the employment of politically correct euphemisms – at times quite unsuccessfully – to mitigate various alleged forms of discrimination.
Incest itself has been pulled both ways, with a few seeking to normalise consensual adult relationships regardless of the degree of kinship (notably in Europe) and others expanding the degrees of prohibited contact (notably in the United States).
In medicine, professionals who practise in ethical and moral grey areas, or fields subject to social stigma such as late termination of pregnancy, might refrain from public discussion of their practice. Among additional reasons, this taboo might come from concern that comments might be taken out of the appropriate context and used to make ill-informed policy decisions.