From what I have learned, this is an exclamation word used to exaggerate what was said. For example: 울고 싶다 > I want to cry 울고 싶단 말이야 > I want to cry a lot. Or I really want to cry. Natives have told me that the word is kind of an older word... maybe not used often in normal conversation (?)

I hear it many times in K-drama

yes ^^

Did you mean "-말이야" ?