A Korean term that means older/big sister that females call a female relative or a older girl they admire or respect. It is sometimes used as a honorific or to replace a person's name.
There are many variations on spelling like Eonnie, Eonni, Unnie, Eunni, Eunnie, or onnie. The male version of this term is Nuna or Noona.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.