What does "bulldog" mean?

Bulldog: (Metal.) A refractory material used as a furnace lining, obtained by calcining the cinder or slag from the puddling furnace of a rolling mill.

Additional senses

  1. 2.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.

Sources