It is a legal term in modern codes. It is not a clinical psychological diagnosis. It is a conceptual tool —a phrase used to name an impossible, terrifying, or paradoxical infinite recursion of kinship.
Now apply that to kinship. A normal family tree is a directed acyclic graph: parents produce children, and the flow goes forward. would represent a cyclic graph —a family tree with a loop. If A gives birth to B, and B then gives birth to A (through time travel or recursive incest), the logical chain breaks. Identity collapses. The very notion of "ancestor" and "descendant" becomes meaningless. incestus ad infinitum meaning