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]