incest
Concise Oxford English Dictionary © 2008 Oxford University Press:
incest/ˈɪnsest/
▶noun sexual relations between people classed as being too closely related to marry each other.
– origin ME: from L. incestus, incestum ‘unchastity, incest’, from in- ‘not’ + castus ‘chaste’.
'incest' also found in these Oxford entries:

