A useful way to hold this word is a door that only opens . Everyone inside deals with everyone inside, and nobody from outside comes in. In the first meaning the thing behind the door is a family, and the closeness is sexual and forbidden. In the second meaning the thing behind the door is a profession or a scene, and the closeness is about work, favours and prizes. The second is a picture borrowed from the first, which is why it always sounds like an accusation and never like praise.
From incest plus -ous, where -ous goes back through Old French to Latin -osus, meaning full of or having the quality of. So the word says having the quality of incest, and the second meaning is that quality used as a picture, not a second history.
由 incest 加上 -ous 組成；-ous 經古法文上溯到拉丁文 -ōsus，意思是「充滿…的、帶有…性質的」。所以這個字字面上就是「帶有亂倫性質的」。第二個意思是把這個性質拿來當比喻用，不是另一條歷史來源。
Wiktionary entry for incestuous, etymology tree incest + -ous, source_ref kaikki:3f6f69a190036ae4:1004811000:12626 (packet evidence)
From the picture
關起門的小圈子（比喻）
同一扇只往內開的門，門後面從一個家換成一個行業或一個圈子。借用字面意思的那份「自己人之間太近」，來說一群人只跟自己人往來。
adjective · C1
Describing a small group whose members only deal with, help, hire and praise one another and keep outsiders out. The person using the word is showing that they do not like it.
小圈圈的
adjective · C1
Involving sex between people in the same family who are too closely related to marry, for example a brother and a sister, or a parent and a child.
亂倫的