Not controlled or restrained; wild and free, often used to describe emotions or behavior that's excessive.
From 'bridle' (a horse's head harness used for control) with the prefix 'un-' meaning not. A horse without a bridle cannot be controlled.
Unbridled literally means a horse that has thrown off its bridle, so when we say someone has 'unbridled passion,' we're imagining a wild horse galloping free—a perfect metaphor for how emotions escape our rational control.
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