The Collaborative International Dictionary of English v.0.48:

Confirmative \Con*firm"a*tive\, a. [L. confirmativus: cf. F.
   confirmatif.]
   Tending to confirm or establish. --Sherwood. --
   Con*firm"a*tive*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

confirmative
    adj 1: serving to support or corroborate; "collateral evidence"
           [syn: collateral, confirmative, confirming,
           confirmatory, corroborative, corroboratory,
           substantiating, substantiative, validating,
           validatory, verificatory, verifying]