1. [n]  a sturdy thickset short-haired breed with a large head and strong undershot lower jaw; developed originally in England for bull baiting
2. [adj]  stubbornly unyielding; "dogged persistence"; "dour determination"; "the most vocal and pertinacious of all the critics"; "a mind not gifted to discover truth but tenacious to hold it"- T.S.Eliot; "men tenacious of opinion"
3. [adj]  resembling or pertaining to a bulldog; "A bulldog appearance"
4. [v]  throw a steer by seizing the horns and twisting the neck, as in a rodeo
5. [v]  attack viciously and ferociously

Bulldog
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1. \Bull"dog`\, n.
   1. (Zo["o]l.) A variety of dog, of remarkable ferocity,
      courage, and tenacity of grip; -- so named, probably, from
      being formerly employed in baiting bulls.
   2. (Metal.) A refractory material used as a furnace lining,
      obtained by calcining the cinder or slag from the puddling
      furnace of a rolling mill.
   

2. \Bull"dog`\, a.
   Characteristic of, or like, a bulldog; stubborn; as, bulldog
   courage; bulldog tenacity.
   {Bulldog bat} (Zo'94l.), a bat of the genus {Nyctinomus}; --
      so called from the shape of its face.