The angle of the lower jaw; in anatomy, the angle formed by the body and ramus of the mandible.
From Greek 'gonia' (angle, corner). Adopted directly into anatomical English from Greek geometry and anatomy terminology, describing a specific junction point in the jaw structure.
Your jaw has corners that anatomists call 'gonia'—it's the same Greek word used for geometric angles, so doctors basically borrowed math language to describe your face.
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