Momentous means of very great importance. A momentous decision, a momentous day, momentous consequences.
It claims that the thing will matter afterwards, which is why it is best used in hindsight and is so often wasted in advance. The moment in it is not the moment of time but the older sense of weight or importance — the same one in momentum and in of great moment.
1adjectivemomentousOf very great importance.
“The fall of the Berlin Wall was a momentous event in European history.”
synonyms: historic, weighty
momentous is a quality of importance.
Latin
mōmentum
Old French
moment
Middle English
moment
English
moment
English, today
momentous
From moment + -ous. moment comes from Middle English moment, from Old French moment, from Latin mōmentum. a doublet of momentum and movement.
Etymology from Wiktionary, CC BY-SA