What is the meaning of -니?

A plain-style interrogative suffix, with a more friendly tone than -냐 (-nya).

With an indirect quote, equivalent to a short form of 고 하니 (-go ha-ni, “does [one] say that...”)

With an indirect quote, equivalent to a short form of 고 하다니 (-go ha-dani, “how can [one] say that...”)

Post-vowel and post-ㄹ (l) allomorphic form of -으니 (-euni, “because, since”)

In an indirect quote, equivalent to a short form of 고 하니 (-go ha-ni, “since [one] says that...”)

colloquial form of -느니 (-neuni)

Post-vowel and post-liquid allomorphic form of -으니 (-euni, alternative form of -느니 (-neuni))

Post-vowel and post-liquid allomorphic form of -으니 (-euni, alternative form of -느니 (-neuni))

Source: wiktionary.org