the.com/incest

A serious violation of trust, often harming the most vulnerable members of a family.

means Sexual relations between people too closely related by blood or family ties to legally or morally wed.

from From Latin incestus, meaning 'unchaste' or 'impure' — itself built from in- (not) plus castus (chaste, pure), the same root that gives us 'chaste' and 'castigate.' So the word originally just meant generic moral filth or impurity before narrowing, over centuries of legal and religious use, to this one specific taboo.

legal status
Illegal in most countries, varying by degree
biology
Increases risk of recessive genetic disorders
royal history
Egyptian and Incan dynasties practiced it for 'pure' bloodlines
word origin
Latin incestus means unchaste or impure
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