We've already talked about Mansae here, but it's always great to review!
Mansae technically means "ten thousand years"
만 - ten thousand 세 - years
It was traditionally used as "대한민국만세" or "May Korea Be for Thousands of Years"
Now its used in two ways:
1. PUT YOUR HANDS UP!
This is used from concerts to doctors offices. Whenever people say raise you hands above your head, they'll say "Mansae!"
2. Thousand Years
It's sort of saying "God Bless so-and-so." For example, if your teacher cancels a test you can say "(teacher's name) mansae!" or when BTS releases shirtless pictures you can say "BIG HIT MANSAE" People generally use it to be funny now :D
Listen to the chorus here to remember "Mansae!"
There are also these adorable triplets named 대한, 민국, and 만세. (dae-han, min-guk, and man-sae)