pertinacity (n.)

"resolute or unyielding adherence," c. 1500, from Middle French pertinacité (early 15c.), from Old French pertinace "obstinate, stubborn," from Latin pertinacem (nominative pertinax) "very firm, tenacious, steadfast, persevering," from per "very" (see per) + tenax (see tenacious). It drove out earlier pertinacy (late 14c.), which was especially "persevering in disbelief."