adj characteristic of, or like, a bulldog; stubborn; as, bulldog courage; bulldog tenacity.
2. bulldog ·noun a refractory material used as a furnace lining, obtained by calcining the cinder or slag from the puddling furnace of a rolling mill.
3. bulldog ·noun a variety of dog, of remarkable ferocity, courage, and tenacity of grip;
so named, probably, from being formerly employed in baiting bulls.