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 the word’s history
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, "incestuous"
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