행복의 열쇠 ― haengbog-ui yeolsoe ― the key to happiness
Etymology
First attested in the Myobeopyeonhwagyeong eonhae (妙法蓮華經諺解 / 묘법연화경언해), 1463, as 엸쇠(Yale: yels.soy). From 열— (yeol-, “to open”) + 쇠 (soe, “metal”). The phonological resemblance to Chinese鑰匙 (yàoshi, “key”) is probably coincidental.