1. What does it mean?
친목 ordinarily means building friendly relationships. Online it can also describe regulars talking mainly among themselves or bringing private friendships into a space meant for open discussion.
Also written or said as chinmok
2. The nuance
The word is not inherently negative. A 친목 모임 exists to socialize, while a community's 친목 금지 rule may aim to prevent cliques; read the local context.
3. Examples
“동아리 첫날에는 친목을 위한 간단한 게임을 했어.”
On the club's first day, we played a simple game to get to know each other.
“새 회원이 소외되지 않게 게시판에서는 친목 대화를 줄이기로 했어.”
We agreed to reduce insider chatter on the board so new members wouldn't feel excluded.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.