Forming a boundary with something or being adjacent to it. Can also mean approaching or verging on a particular state or condition.
From 'border', which comes from Old French 'bordure', derived from 'bord' (edge, side). The verb form developed in the 16th century, extending from physical boundaries to metaphorical ones like 'bordering on madness'.
The word beautifully captures both physical and metaphorical boundaries - you can border a country or border on genius. Medieval manuscripts had elaborate decorated borders, which is why we still call decorative edges 'borders' today.
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