1. What does it mean?
찐친 combines 찐, genuine, with 친구, friend. It emphasizes a real sense of closeness, comfort, or trust rather than a casual acquaintance.
Also written or said as 찐 친구, jjinchin
2. The nuance
It does not have to identify a single best friend, and there is no required number of years of friendship. People may use it affectionately in captions, but the label does not make every joke or boundary-crossing acceptable.
3. Examples
“오랜만에 만나도 어색하지 않은 걸 보니 역시 찐친이야.”
It's not awkward even after all this time. We really are close friends.
“찐친이어도 약속을 바꾸면 미리 알려 줘야지.”
Even close friends should let each other know beforehand when plans change.
Sources
Financial News: Korean language in use
References support the meaning and usage. Example sentences are written for NoCapWiki.