1. What does it mean?
Shortened from 인사이더, the Korean form of 'insider,' 인싸 describes someone who mixes easily with people or is well connected within a social circle.
Also written or said as 인사이더, inssa
2. The nuance
It is a loose social label, not a personality test. Knowing everyone at one club does not mean someone is outgoing everywhere. Among friends it can be admiring or teasing; it should not imply that less sociable people are inferior.
3. Examples
“새 동아리에 아는 사람이 벌써 많네. 완전 인싸다.”
You already know so many people in the new club. You're really well connected.
“인싸처럼 보이지만 혼자 쉬는 시간도 좋아해.”
I may seem like a social butterfly, but I enjoy time alone too.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.