One who chins (performs chin-ups or hangs by the chin), or one who touches, grasps, or works with the chin area.
From 'chin' (to grasp with the chin, or to do a chin-up) plus '-er' agent suffix, creating a term for someone performing this action.
In gymnastics and fitness history, a 'chinner' was someone doing chin-ups before that exact term became standard—old training manuals have wonderfully specific vocabulary for exercises we now call by different names.
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