maybe it was Egyptian http://parsha.blogspot.com/2009/06/who-in-world-is-nun.html

@rosends, I highly appreciate the explanation. In fact, it may even explain why Nun is so often mentioned along with Yehoshua. I picture a scenario around something like: Elishama had lost hope of leaving Egypt, and so decided to at least attempt to prepare his son for assimilation into Egypt. His son, however, didn't lose hope and so returned to the traditional Hebrew name with his son. For this, he is commended by the Tanach. Just a סברא, though.

Take care with citing Midrashim as to the etymology of names; by this logic you'd come to conclusions such that even Chavah had an Aramaic name (Bereishis Rabbah 20:11), even though Aram wouldn't exist for another 12 generations and Aramaic for another 15.

@DonielF I guess that's true. Unfortunately, I have yet to find a Hebrew meaning for Nun (other than the letter...). Any suggestions would be welcomed.

Related: https://judaism.stackexchange.com/questions/20251/who-was-yehoshua-bin-nuns-father

It seems R' Ahron Marcus in Barzilai believes that Nun was originally a Hebrew word. He also says that Nun per the Egyptian meaning came from one of the Babylonian-Hebraic dialects.