• Etymology Today from M-W: pertinacious \per-tuh-NAY-shuss\

1 a : adhering resolutely to an opinion or purpose b : perversely persistent
2 : stubbornly unyielding or tenacious

If you say "pertinacious" out loud, it might sound familiar. That may be because if you take away the word's first syllable, you're left with something very similar to the word "tenacious," which means "tending to adhere or cling." The similarity between "pertinacious" and "tenacious" isn't mere coincidence; both words derive from "tenax," the Latin word for "tenacious," and ultimately from the verb "tençre," meaning "to hold." But "pertinacious" and "tenacious" aren't completely interchangeable. Both can mean "persistent," but "pertinacious" suggests an annoying or irksome persistence, while the less critical "tenacious" implies strength in maintaining or adhering to something valued or habitual.

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