Stubborn
WordNet
adjective
(1) Tenaciously unwilling or marked by tenacious unwillingness to yield
(2) Not responding to treatment
"A stubborn infection"
"A refractory case of acne"
"Stubborn rust stains"
WiktionaryText
Etymology
From , , , ; probable origin *stybor, *stibor, apparently from + adj. formative as in bitor, E. bitter.