/ ɪnˈsestjuəs; NAmE -tʃuəs/ adjective
1.
involving sex between two people in a family who are very closely related :
an incestuous relationship
2.
( disapproving ) involving a group of people who have a close relationship and do not want to include anyone outside the group :
The music industry is an incestuous business.
► in·ces·tu·ous·ly adverb
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WORD ORIGIN
early 16th cent.: from late Latin incestuosus , from Latin incestus , incestum unchastity, incest, from in- not + castus chaste.