British slang for hair, derived from Barnet Fair rhyming slang.
From Cockney rhyming slang 'Barnet Fair' meaning 'hair', named after the famous horse fair held in Barnet, North London since the 12th century. The rhyming element was eventually dropped, leaving just 'barnet'.
The fair connection makes perfect sense when you consider that horses were judged partly by their manes - their 'hair' - making the leap from horse fair to hair almost logical in the twisted genius of Cockney rhyming slang.
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