Lines that mark boundaries or borders; also used to describe cases or situations that are on the edge of fitting into a category (borderline cases).
From 'border' plus 'line,' creating a compound for literal boundary demarcations. The metaphorical sense developed to describe things that are questionable or marginal in nature.
Borderlines is interesting because it works both literally (actual lines on maps) and psychologically (borderline personality disorder)—showing how geography metaphors permeate how we think about identity and classification.
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