From Longman Dictionary of Contemporary Englishbulldogbull‧dog /ˈbʊldɒɡ $ -dɒːɡ/ noun [countable] 
HBAa powerful dog with a large head, a short neck, and short thick legs
HBAa powerful dog with a large head, a short neck, and short thick legsExamples from the Corpus
bulldog• Like a bulldog it refuses to let go, even when savagely attacked by the desperate victim.• The beefy chest of a bulldog.• He reminded Harry, in his build, expression and asthmatic wheeze, of a bulldog peering ill-humouredly from his kennel.• Want to show a bulldog rolling its eyes?• Both bite their attackers and hang on like bulldogs, letting the venom trickle into the wound as they do so.• Sadly, this right to leave your bits and pieces to your budgie or bulldog is denied those with hereditary titles. 