A Korean term that means older/big sister that females call a female relative or a older girl they a admire or respect. It is sometimes used as a honorific or to replace a person's name.
There are many variations on spelling like Unni, Eonni, Eonnie, Eunni, Eunnie, or onnie. The male version of this term is Nuna or Noona.
Unnie is the what you call and female who is older than you and you are close with them. THis word is from korean culture so everyone who is a K-Pop stan MUST know this word.