cocksure

Probably from cock +‎ sure, early 16th c.

cocksure (comparative more cocksure, superlative most cocksure)

1. (informal) too confident; overconfident

   I thought myself cocksure of the horse which he readily promised me. — Alexander Pope

   • 1906, John Galsworthy, “preface”, in The Man of Property:

     The persistence of the Past is one of those tragicomic blessings which each new age denies, coming cocksure on to the stage to mouth its claim to a perfect novelty.

   • 1920, Sinclair Lewis, “XXIII”, in Main Street:

     These crack specialists, the young scientific fellows, they're so cocksure and so wrapped up in their laboratories that they miss the human element.