IPA: /məˈmɛn.təs.li/
KK: /moʊˈmɛntəsli/
In a way that is very important or significant, often relating to events that have a major impact.
The decision was made momentously, changing the course of history.
Momentously → It is formed from "moment" (from Latin *momentum*, meaning movement or importance) and the suffix "-ously" (meaning in a manner of). The word "momentously" means in a manner that is significant or important, often relating to a critical moment.
Think of 'moment' as something that carries 'importance' — when something happens 'momentously', it means it is significant or crucial.