This is the 13321st most frequent Korean word.
등산하자고
"Let's go hiking."
The term '등산하자고' is used to express a suggestion about hiking.
오늘 날씨가 좋으니 등산하자고 제안했어요.
Since today's weather is good, I suggested going hiking.
Here, '등산하자고' represents the proposal made collectively by the friends.
친구들이 모두 등산하자고 했어요.
My friends all suggested going hiking.
This usage of '등산하자고' indicates the speaker's thought on the act of suggesting a hike.