1650s, from moment + -ous to carry the sense of "important" while momentary kept the meaning "of an instant of time." Related: Momentously; momentousness.
实用例句
1. In that year, two momentous discoveries were made.
那一年，有两个重大发现。
来自柯林斯例句
2. I am deeply honoured to be invited to this momentous occasion.
能应邀出席如此重要的场合,我深感荣幸.
来自《简明英汉词典》
3. The momentous news was that war had begun.
重大的新闻是战争已经开始.
来自《简明英汉词典》
4. Meanwhile, most young people in the West are expected to leave what could be life's most momentous decision—marriage—almost entirely up to luck.
同时，人们认为西方的大多数年轻人把婚姻这一可能是人生最重要的决定几乎完全交由命运来安排。
来自柯林斯例句
5. The past three years have been among the most momentous in world history.