incest

 
Pronunciation: /ˈɪnsɛst/

noun

[mass noun]
  • sexual relations between people classed as being too closely related to marry each other.
  • the crime of having sexual intercourse with a parent, child, sibling, or grandchild.

Origin:

Middle English: from Latin incestus, incestum 'unchastity, incest', from in- 'not' + castus 'chaste'