1. What does it mean?
ㅊㅋ is a chat abbreviation associated with 축하, pronounced roughly chu-ka. Friends use it to celebrate news such as an acceptance, birthday, or personal achievement.
Also written or said as ㅊㅋㅊㅋ, 축하, chuka
2. The nuance
The ㅋ reflects the sound of 축하 rather than a strict letter-for-letter initial-consonant rule. It suits a quick friendly reaction; a fuller message is better when you want the congratulations to feel personal.
3. Examples
“첫 전시 열었다며? ㅊㅋ! 주말에 보러 갈게.”
I heard you opened your first exhibition. Congrats! I'll go see it this weekend.
“면허 땄구나, ㅊㅋㅊㅋ. 연습한 보람 있네.”
You got your license—congrats! All that practice paid off.
Sources
FluentU: Korean text slang guide
References support the meaning and usage. Example sentences are written for NoCapWiki.