Yes. I think It's slang. The word '도른자(crazy people)' is a new word. 돌은 + 자 = 돌은 자 -> 도른 자   '돌은' is turn or go nuts adjectives. '자' is persion noun. '돌은 자' and '도른 자' Though phonetically alike,they are written with differentideographs. * Bats in the belfry This means that someone is very eccentric or even crazy.

Maybe you heard 노른자 not 도른자. 노른자 means egg yolk. In many cases, 노른자 means the essence, or highest-priced land of an area, heart, kernel, etc.