잖냐 is shortened form of 지 않냐(지 아니하냐) ~지 않다 is negative form of ~다 냐 is interrogative ending suffix used between intimate friends. For example, 지금 가잖냐 = 지금 가지 않냐? (I'm going now, right?) 너 어제 먹었잖냐 = 너 어제 먹었지 않냐? (You ate it yesterday, didn't you?)