AUTHORITATIVELY
authoritatively, magisterially
(adverb) in an authoritative and magisterial manner; “she spoke authoritatively”
Source: WordNet® 3.1
Etymology
Adverb
authoritatively (comparative more authoritatively, superlative most authoritatively)
In an authoritative manner, being worthy of trust.
Source: Wiktionary
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: Webster’s Unabridged Dictionary 1913 Edition