Done or happening at a dangerously fast speed; recklessly fast or rapid.
From 'break' + 'neck'; literally meaning fast enough to break someone's neck. The term emerged in the 16th-17th centuries as a vivid way to describe speed that could cause fatal injury.
Medieval and Renaissance travelers actually warned about 'breakneck' speeds on horseback—going too fast downhill could literally cause a fatal broken neck. The metaphor stuck because the danger was real and immediate.
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