Here you go: "A korean's way of calling a close girl part of their family, much more like an older sister(in a girls view)." http://www.urbandictionary.com/define.php?term=unnie So it's a Korean word.

"Onni" (언니 - literally, "older sister") is a form of address used when a Korean girl addresses her older sister, or another woman who is older than her (and therefore has a little more status). A younger man would use "nuna" (누나) when addressing the same woman.

I think you might mean 'uni', in which case it's short for 'university'. It's hard to tell without context. Where did you hear the word?

What was the context? What was the rest of the sentence? We can't guess the word without something to go by.

I heard it when i was watching a korean drama ..