1. What does it mean?
ㄷㄷ evokes 덜덜, the sound-symbolic expression for trembling. In text it can convey alarm, disbelief, or being impressed by something extreme.
Also written or said as ㄷㄷㄷ, deol-deol
2. The nuance
It does not always mean actual fear. Context distinguishes an intimidating price from an astonishing achievement, and repeated ㄷ characters intensify the visual reaction.
3. Examples
“계단을 쉼 없이 올라갔다고? ㄷㄷ 체력 좋네.”
You climbed all those stairs without stopping? Whoa, impressive stamina.
“바람 소리 ㄷㄷ. 창문 닫아야겠다.”
That wind sounds intense. I'd better close the window.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.