What is the meaning of -다니?

In the intimate style or in monologues, used to express surprise or anger at something one has just learned or remembered. Often followed by a sentence expanding on the emotion being felt.

Used other than figuratively or idiomatically: see -다 (-da),‎ -니 (-ni) — equivalent to a short form of 다 고 하 니 (-da-go ha-ni)

Source: wiktionary.org