dead certain
Absolutely sure. I'm dead certain that the answer is 42, but you can check the answer key if you want. Elise was dead certain that the wedding started at four, and, had I listened to her, I would have gotten there on time!
dead certain
very sure. (Dead means absolutely.) I'm dead certain that horse will win. I bet two hundred on it myself. I didn't believe the rumor at first, but Bill's dead certain that it's true.