Historically, "pit bull" was an informal and slang label used to describe any dog that was subjected to the cruel sport of bull-baiting - using dogs (of any breed) to seize tethered animals such as
What is a Pit Bull?
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Pit bull - Wikipedia
What's a pit bull? Animal Humane Society
What is a pit bull? It's not actually a dog breed
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Battling A Bad Reputation: Why Do So Many People Dislike Pit Bulls? - Faunalytics
Pit Bull Is Not a Breed
What's a pit bull? Animal Humane Society