1. What does it mean?
As a label derived from 아웃사이더 ('outsider'), 아싸 refers to someone outside a group's social circle. Depending on context, the identical spelling can instead be an exclamation of delight.
Also written or said as 아웃사이더, assa
2. The nuance
나는 아싸야 describes a social position; 아싸, 성공했다! celebrates success. The outsider label can be self-deprecating, chosen, or hurtful when imposed on someone. It does not establish that a person is lonely or wants more friends.
3. Examples
“큰 모임에서는 아싸지만 친한 친구들이랑은 말이 많아.”
I'm on the sidelines in big groups, but I talk a lot with close friends.
“아싸, 취소표가 나와서 공연에 갈 수 있겠다!”
Yes! A ticket opened up, so I can go to the show!
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.