authoritatively

From Middle English auctoritatively, auctoritativeli; compare Medieval Latin auctoritātīve; equivalent to authoritative +‎ -ly.

authoritatively (comparative more authoritatively, superlative most authoritatively)

In an authoritative manner; with authority.
- 1888, Rudyard Kipling, “The Phantom Rickshaw”, in The Phantom 'Rickshaw and Other Tales, Allahabad: A.H. Wheeler and Co., page 8:
  He has, of course, the right to speak authoritatively, and he laughs at my theory that there was a crack in Pansay's head and a little bit of the Dark World came through and pressed him to death.
- 2017 May 16, Dylan Byers, “Does anyone in Trump's White House have credibility?”, in CNN Business‎:
  The inability of Trump's own spokespeople to speak authoritatively on the president's thinking and actions -- a challenge even Trump himself has acknowledged -- has left the White House with an enormous credibility gap, and has tarnished the reputations of many of its most visible spokespeople.

in an authoritative manner