단

Shortened 'that', in 단 말이야

A squeeze of 다는, 'that (someone said)'. In practice you meet it in exactly one place: every one of these eight lines follows it with 말 or 얘기. That whole phrase means 'the fact that...', and used as a sentence ending it presses your point home, roughly 'I'm telling you' or 'and that's the thing'. ㄴ단 and 는단 are the same construction attached to present-tense action verbs.

Register: conversational