Past tense of 'buck': to resist or oppose something, or for a horse to jump with its back arched.
From Old English 'bucc' meaning male goat or deer. The horse-bucking meaning comes from observing how these animals throw their bodies upward when resisting riders, around the 1600s.
Horses buck as an instinctive defense mechanism—it's the same motion they'd use to dislodge a predator, which is why wild horses buck far more violently than domesticated ones that have learned to tolerate riders.
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