던데

I heard or saw that..., and

Passes on something you saw or were told, then keeps going into what you want to say about it: there's a rumour you go back further than that, so lend me some money, I still see her posting in that community, I kinda feel bad. The 더 part means the information came to you first-hand or by report; the 는데 part keeps the sentence open for the point you are building to. Compare 더라, which reports the same kind of thing and then stops.

Register: casual