언니
Eonni
What does it mean?
A woman uses 언니 for her older sister. It can also be used for an older female friend or another woman she is close to.
How it sounds
A: 언니, 오늘 뭐 해? B: 나 오늘 쉬어.
When do Koreans use it?
The speaker is female and the other woman is older. It usually signals familiarity and closeness.
A small nuance
A man would normally use 누나 for an older sister or an older woman he is close to.