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