A dog (especially a herding dog like a cattle dog or sheep dog) that herds livestock by nipping at their heels; also can refer to a person who heels or attacks with the heel.
From 'heel' plus the agent suffix '-er,' meaning 'one who heels' or applies pressure with the heel.
Australian cattle dogs are often called 'heelers' because they were bred specifically to drive cattle by nipping at their heels—it's a unique herding technique that works because the heel is one of the few places livestock can't kick back effectively.
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