1. What does it mean?
ㄲㅂ abbreviates 까비, itself related to 아까비. It reacts to a missed chance, a narrow failure, or something that nearly worked out.
Also written or said as 까비, kka-bi
2. The nuance
It is often light sympathy, not a statement that someone performed badly. A brief ㄲㅂ may feel insufficient when the loss is genuinely serious.
3. Examples
“마지막 공 하나가 안 들어갔네. ㄲㅂ.”
That last shot didn't go in. So close.
“매진 직전에 놓쳤다니 ㄲㅂ. 다음 공연 노려 보자.”
You just missed the sellout? That's a shame. Let's try for the next show.
Sources
National Institute of Korean Language
References support the meaning and usage. Example sentences are written for NoCapWiki.