Mo*men"tous*ly

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]