make book

The "be confident" sense is connected to the "gamble" sense: a sure bet is one that you can be nearly certain of.

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make book (third-person singular simple present makes book, present participle making book, simple past and past participle made book)

1. (idiomatic) To gamble, either by placing or taking bets.
   • 1954 August 30, “Into the Stretch”, in Time:

     In the American League, front-running Cleveland was an odds-on favorite with the men who make book on baseball.

   • 1962, Stanley Shapiro, Nate Monaster, That Touch of Mink:

     [Connie:] How is Mrs Miller?
     [Mr Miller:] She doesn't understand me.
     [Connie:] I could have made book on that answer.

2. (idiomatic) To be very confident; to be sure.

   Near-synonym: bet one's bottom dollar

   • 1909, Rex Ellingwood Beach, chapter 25, in The Silver Horde:

     "Oh, you can make book that she didn't start no love-making. She ain't the kind to curl up in a man's ear and whisper."