from Latin magnificus magnific + -ous
E-NED only gives an example for the adverb, 'magnificously' :-
1683 - Edward Hooker, Prefix in John Pordage's 'Theologica Mystica, or The Mystic Divinitie of the Æternal Invisibles:-
"How magnificously soever wee bragg..."
From: The History of the College of Corpus Christi and the B. Virgin Mary,
(Commonly Called Benet) in the University of Cambridge.
From its Foundation to the Present Time.
By Robert Masters, 1753