Chingu means friend.
However, like other Korean words, they often aren’t used the same way we would. Chingu, in Korea, is usually used only among same age friends (unless they come to an agreement) as they use other terms to relate with friends who are of a different age.
These friendship terms depend on gender, age, and if you have received permission from the older member to use less formal language. If you are male you would use “hyeong” (형) for an older male friend and ”nuna” (누나) for older female friend. If you are female you use “oppa“ (오빠) for an older male friend and eonni (언니). A younger friend can be called dongsaeng (동생) by either gender.
You may have heard it being used in dramas to identify the close relationship of boyfriend - namja chingu (남자 친구) or girlfriend - yeoja chingu (여자 친구).
This clip from KBS’s Happy Together 2017.09.07 shows SHINee’s Taemin and EXO’s Kai explain how it can be tricky to become a chingu. (Full episode can be seen on YouTube at KBS World
They have certainly moved beyond the awkwardness of their middle school days and now have a deep bond as chingus.
They can be seen performing together as part of SM Performance, SM Town and most recently SuperM.
Scroll over for pictures of their friendship.