Hot-tempered, rash, or impulsive in disposition.
From Middle English 'cholter' (anger, hot temper) derived from 'chol' (relating to choler/bile). Medieval medicine attributed hot temper to excess yellow bile (choler), so 'cholter-headed' meant excessively bilious or angry.
Medieval doctors literally thought your personality was controlled by the balance of four bodily fluids, so a 'cholterheaded' person was someone with too much yellow bile—today we'd just say they're hot-headed!
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