Involving inappropriate sexual relations within a family, or more broadly, describing something that's overly self-referential or closed to outside influence.
From Latin 'incestus,' from 'in-' (not) plus 'castus' (pure). The biological sense is ancient, while the metaphorical sense (self-referential systems) developed in modern usage.
The word comes from Latin meaning 'not pure,' but 'incestuous' is now used to describe anything too insular, like 'incestuous academic circles' where scholars only cite each other. The metaphor reveals how we use family taboos to discuss group insularity.
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