Sullenly obstinate; obstinately determined or persistent; as, dogged resolution; dogged work; dogged pursuit.

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"

Sullen; morose.