Incestuous means involving sex between close relatives. An incestuous relationship, incestuous marriage, the incestuous plot.
It is illegal everywhere and it is a serious crime where a child or a dependent is involved. Figuratively it describes a closed world where the same few people deal only with each other — an incestuous industry — and that use is common and much milder. Note incest beneath it.
1adjectiveincestuousInvolving sex between close relatives.
“The novel caused a scandal because it described an incestuous love affair between a brother and sister.”
2adjectiveclosedClosed and inward-looking.
“The village theatre group was rather incestuous, and newcomers never got the good parts.”
incestuous is a quality of relation.
Latin
incestus
English
incest
English, today
incestuous
From incest + -ous. incest Latin incestus. mǣġhǣmed. a doublet of inchaste.
Etymology from Wiktionary, CC BY-SA