んだってI heard that, He or She said that

When used at the end of sentences, だって often expresses something that has been heard by the speaker. This particular use of だって will always be paired with ん or なん.

As だって is a grouping of the auxiliary verb だ, and the adverbial particle って, なんだって primarily expresses the following:

なん - Presents an explanation.

だ - Determines something to be true.

って - An abbreviation of と言(い)って 'is said' or と思(おも)って 'is thought'.

In this way, it translates quite similarly to 'it is that (A) was said/thought'. Realistically though, it is used in the same way as 'I heard that (A)', or 'it's thought that (A)'.

Caution

なんだって usually comes across as being quite casual, so is best used in conversations with friends.