1 Tending to keep a firm hold of something; clinging or adhering closely.
1.1 Not readily relinquishing a position, principle, or course of action; determined.
1.2 Persisting in existence; not easily dispelled.
Early 17th century: from Latin tenax, tenac- (from tenere ‘to hold’) + -ious.
Thomas was tenacious about his coffee mug. He washed it carefully every day and sequestered it in his own desk drawer. Nobody else in the office should use it. Otherwise, people just grabbed the closest cup on the shelf in the break room.