- bulldog, dogged, dour, pertinacious, tenacious, unyielding - stubbornly unyielding; "dogged persistence"; "dour determination"; "the most vocal and pertinacious of all the critics"; "a mind not gifted to discover truth but tenacious to hold it"- T.S.Eliot; "men tenacious of opinion"
Antonym: docile (indirect, via stubborn)