pertinacious
1. [ adjective ] stubbornly unyielding
Synonyms: dogged tenacious dour unyielding bulldog
Examples:

"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"

Related terms: stubborn
Similar spelling:   pertinaciously
  pertinacity