A large dog breed with an extremely powerful sense of smell, historically used to track people or prey by scent; also means someone obsessed with finding something.
From 'blood' (Germanic) plus 'hound' (Old English 'hund'). The breed was developed in medieval Europe, specifically associated with tracking fugitives and hunting game.
Bloodhounds have noses so sensitive they can follow a scent trail over 300 hours old and across dozens of miles—they've literally cracked criminal cases by tracking suspects. Their tracking ability inspired the metaphor for relentless investigators.
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