Source : Webster's Revised Unabridged Dictionary (1913)

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

            The sacred functions of authoritative teaching.
                                                  --Barrow.

   2. Having an air of authority; positive; dictatorial;
      peremptory; as, an authoritative tone.

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

Source : WordNet®

authoritatively
     adv : in an authoritative manner; "she spoke authoritatively"
           [syn: {with authority}, {magisterially}]