Source : Webster's Revised Unabridged Dictionary (1913)

Momentous \Mo*men"tous\, a. [Cf. L. momentosus rapid,
   momentary.]
   Of moment or consequence; very important; weighty; as, a
   momentous decision; momentous affairs. -- {Mo*men"tous*ly},
   adv. -- {Mo*men"tous*ness}, n.

Source : WordNet®

momentously
     adv : in a momentous way