a.1.Guilty of incest; involving, or pertaining to, incest; as, an incestuous person or connection.
Ere you reach to this incestuous love,
You must divine and human rights remove.
- Dryden.
2.resembling incest; between people or groups so closely related as to make such activity improper or unethical; as, an incestuous coziness between the management of the company and its union; an incestuous relationship between the salesman and the purchasing manager.
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.