The Collaborative International Dictionary of English v.0.48:

Authoritative \Au*thor"i*ta*tive\, a.
   1. Having, or proceeding from, due authority; entitled to
      obedience, credit, or acceptance; determinate; commanding.
      [1913 Webster]

            The sacred functions of authoritative teaching.
                                                  --Barrow.
      [1913 Webster]

   2. Having an air of authority; positive; dictatorial;
      peremptory; as, an authoritative tone.
      [1913 Webster]

            The mock authoritative manner of the one, and the
            insipid mirth of the other.           --Swift.
      [1913 Webster] -- Au*thor"i*ta*tive*ly, adv. --
      Au*thor"i*ta*tive*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

authoritatively
    adv 1: in an authoritative and magisterial manner; "she spoke
           authoritatively" [syn: authoritatively,
           magisterially]