A conceited or arrogant person; also, a type of freshwater fish with a large, bulbous head.
Compound of English 'big' + 'head', literal compound dating to at least the 16th century. Applied metaphorically to arrogant people, and later as a common name for various fish species with enlarged craniums.
Bighead fish were named purely for their appearance, but the term stuck so well that 'bighead' became a primary common name used in fisheries science. Meanwhile, the metaphorical meaning—arrogant person—shows how physical features have been tied to personality traits across cultures for thousands of years.
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