A saying-back that establishes a thing was received and is so. Confirmation is what turns an assumption into a shared fact.
Latin confirmatio (a strengthening, confirmation), from confirmare (to make firm), from com- (thoroughly) + firmare (to strengthen), from firmus (firm); the noun for the act of establishing that a thing was received and stands as true.
Latin firmus is firm, and confirmation was always about making something solid rather than merely repeating it back. An assumption stays private until it is made firm this way; only then does it belong to both sides equally.