1. What does it mean?
ㅈㅅ comes from the consonants of 죄송. It is a compressed apology, often for a small mistake made while chatting or playing with other people.
Also written or said as ㅈㅅㅈㅅ, 죄송, joesong
2. The nuance
It can feel perfunctory even though 죄송 is part of polite apologies. When harm is serious or the other person is upset, writing only ㅈㅅ may make the apology seem dismissive.
3. Examples
“다른 채팅방에 보낼 걸 여기 보냈네. ㅈㅅ, 무시해 줘.”
I sent that to the wrong chat. Sorry, please ignore it.
“ㅈㅅ 마이크 켜진 줄 모르고 과자 먹었어.”
Sorry, I was eating chips without realizing my microphone was on.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.