The Collaborative International Dictionary of English v.0.48:

Corroborative \Cor*rob"o*ra*tive\ (k?r-r?b"?-r?-t?v), a. [Cf. F.
   corroboratif.]
   Tending to strengthen of confirm.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Corroborative \Cor*rob"o*ra*tive\, n.
   A medicine that strengthens; a corroborant. --Wiseman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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