Source : Webster's Revised Unabridged Dictionary (1913)

Bulldog \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.