Great word.

pertinacious (pur-tin-AY-shuhs) adjective

1. Holding resolutely to a purpose, belief, or opinion.

2. Stubbornly unyielding.

[From Latin pertinac- pertinax, per- (thoroughly) + tenax (tenacious),
from tenere (to hold).]

Today's word in Visual Thesaurus: http://visualthesaurus.com/?w1=pertinacious

-Anu Garg (garg wordsmith.org)

"A man is pertinacious when he defends his folly and trusts too greatly
in his own wit."
Geoffrey Chaucer; Canterbury Tales: Explicit Secunda Pars Penitentie;
1387-1400 (Translation: Walter W. Skeat).