눈치가 백단이야.
What does 백단 mean? Is this phrase an idiom?

Question

Answer

백단 is a kind of exaggeration to discribe someone's East Asian board game skill level.
This expression is same as 20,000 rate point in chess. Haha.
As you know, a champion of chess competition can read opponents' mind very well.
So if someone's 눈치 is 백단, this one can read other's mind very well like a champion of a chess competition.

Answer

백단(100단; 바둑이라는 동양의 게임/스포츠의 최고 등급이 10단. 그보다 아주 높은 수준).
눈치: wits, sense
'눈치가 백단이다'; Someone really catches on fast. A colloquial idiom.
"영어도 못 하면서 미국에서 살다 보니, 눈치가 백단이 되었어요."
"쟤는 눈치가 백단이야. 정말 여우 같아."