Older sister — said only by a woman.
A woman calls any older woman 언니 — sister, friend, a favourite shop owner, an older coworker. It is warm and used constantly. Shop staff will even call customers 언니 to be friendly.
The female counterpart to 누나, completing the pair that 오빠 and 형 make for men.
Only from a woman. A man saying 언니 is a well-known beginner slip — he needs 누나.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to Korean — it uses 언니 the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook Korean.
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