Resistant to or incapable of being controlled by a halter; wild or unmanageable.
From halter combined with proof (meaning resistant to, from Old French 'prover' meaning to test), following the pattern of waterproof, dustproof, etc.
The -proof suffix emerged from testing practices—if something was 'proof against' water, it could withstand being tested in water. Now it's generic for 'resistant to anything.'
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