The Collaborative International Dictionary of English v.0.48:

Adversative \Ad*ver"sa*tive\, a. [L. adversativus, fr.
   adversari.]
   Expressing contrariety, opposition, or antithesis; as, an
   adversative conjunction (but, however, yet, etc. ); an
   adversative force. -- Ad*ver"sa*tive*ly, adv.
   [1913 Webster]