bulldog

Concise Oxford English Dictionary © 2008 Oxford University Press:
bulldog/ˈbʊldɒɡ/

▶noun

• 1 a dog of a sturdy breed with a powerful protruding lower jaw, a flat wrinkled face, and a broad chest.

• 2 informal (at Oxford and Cambridge Universities) a proctor's assistant.

▶verb (bulldogs, bulldogging, bulldogged) N. Amer. wrestle (a steer) to the ground by holding its horns and twisting its neck.

– derivatives
bulldogger noun.