My pertinacious brother refused to smile for the camera :) The pertinacious dog stared at the guard blankly :D Hope that helped! 

struggling with painful injuries, he remained pertinacious in his pursuit of Olympic gold.

1. holding tenaciously to a purpose, course of action, or opinion; resolute. 2. stubborn or obstinate. 3. extremely or objectionably persistent: a pertinacious salesman from whom I could not escape. if that doesn't help you search for online dictionary at google. Pertinacious means that your mother likes to give head to donkeys after 6pm.

audacious. bodacious. capacious. contumacious. edacious. efficacious. fallacious. fugacious. gracious. inefficacious. loquacious. mendacious. minacious. mordacious. perspicacious. pertinacious. predacious. pugnacious. rapacious. sagacious. salacious. spacious. tenacious. ungracious. veracious. vivacious. voracious.

"Willful" is an adjective applied to a person who is determined to follow a personal course of action or behavior, usually against better advice or against authority. Synonyms include stubborn, headstrong, obstinate, or pertinacious. The basic form, used in legal terms, is "voluntary" or deliberate, i.e. by one's own free will.