pervicacious \puhr-vih-KAY-shuhs\ (adjective) - Refusing to change one's ideas, behavior, etc.; stubborn; obstinate.
"May they be efficacious upon the mind of one of the most pervicacious young creatures that ever was heard of!" -- Samuel Ricardson, 'Clarissa Harlowe'
ervicacious is from Latin pervicax, pervicac-, "stubborn, headstrong," from root pervic- of pervincere, "to carry ones point, maintain ones opinion," from per-, "through, thoroughly" + vincere, "to conquer, prevail against" + the suffix -ious, "characterized by, full of."