The story
From bull + dog, in reference to bullbaiting.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: A breed of dog developed in England by the crossing of the bullbaiting dog and the Pug to produce a ladies' companion dog, having a very smooth coat, a flattened face, wrinkly cheeks, powerful front legs, and smaller hind legs. Full definition →
From bull + dog, in reference to bullbaiting.
Etymology from Wiktionary, CC BY-SA 4.0.