1495-1505; < Late Latinpertinācitās, for Latinpertinācia stubbornness, perseverance (> obsolete pertinacy), equivalent to pertināci- (stem of pertināx) steadfast, stubborn + -tās-ty2. See per-, tenacity
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.).