건 모르다

don't know, or don't care

Two different things share this spelling. In five lines 이건 몰라 is simply 'this one I don't know', where 이건 is 'this thing'. The other is X건 Y건 모르겠다 — whether it's forty or fifty percent, whether it's two degrees or three, I have no idea and it makes no difference — a dismissal of the whole range at once, and 올리건 말건 몰라 is the same shrug.

Register: casual; the second use is dismissive