Green’s Dictionary of Slang

1. large and potentially violent and/or threatening.

1932		S. Brown ‘Puttin’ On Dog’ in Botkin Folk-Say 254: With bulldog brogans red as a clay road.
1966		Trimble 5000 Adult Sex Words and Phrases 32: bulldog dose (Vulg.) A severe case of a venereal Disease, usually Gonorrhea.
1988		D. Woodrell Muscle for the Wing 135: I want something [i.e. a weapon] seriously bulldog in my hands when I come through that door.
2000		C. Cook Robbers (2001) 215: Two bulldog dykes in work-boots and flannel shirts giving her the eye.

2. uncompromising, indomitable.

1928		J. Callahan Man’s Grim Justice 122: I had only a bulldog faith in myself and plenty of guts.
1961		W.R. Burnett Conant 50: Nothing against him I know of. No strong-arm stuff. Just bulldog persistence.
2012		S. Blass Pirate for Life 152: [Dave] Giusti, our bulldog closer .