To train a horse to accept and respond to a halter for the first time.
From halter (a strap around a horse's head) and break (to train). This is a compound unique to horsemanship vocabulary, emerging in English during the agricultural period.
Halter-breaking is one of the first lessons a foal learns, teaching submission to human control. The term itself is beautifully direct: you're literally teaching the horse to accept being 'broken' to the halter.
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