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1530s, from Latin incestuosus "incestuous," from incestus "unchaste" (see incest). Figurative use is from 1744. Related: Incestuously; incestuousness.
incestuous in·ces·tu·ous (ĭn-sěs'chōō-əs)adj.
Of, involving, or suggestive of incest.
Having committed incest.